Calling norns(ctl)?_error() to query the status of a running task will
release its descriptor in the server if the task already completed
(NORNS_EFINISHED or NORNS_EFINISHEDWERROR). Since norns(ctl)?_wait()
internally queries the task status to check whether a task has
completed, we cache the retrieved task_status into the iotask_t user
descriptor so that it can be retrieved by a subsequent
norns(ctl)?_error() call.

When invoked, the daemon checks whether active tasks remain to be
processed and, if so, returns NORNS_ETASKSPENDING. If there are no
active tasks, but there are tracked namespaces that are not empty, the
daemon return NORNS_ENAMESPACENOTEMPTY. Otherwise, the daemon returns
NORNS_SUCCESS and shuts itself down.

Two new private member functions pause_accept() and resume_accept() have
been added to urd, which internally use the compare_exchange_weak()
method to atomically test and set m_is_paused.

Fixes #15.

Clients can now provide a 'track' flag when defining a namespace to let
urd know that it must keep track of the namespace contents. The flag is
currently propagated by the messaging system and stored in the internal
backend metadata. Tracking not implemented yet.

The new nornctl_status function queries the urd daemon and returns
summarized stats about all iotasks currently in the system. More
specifically, the function returns a nornsctl_stat_t structure that
contains the following information:
    st_running_tasks: Number of tasks currently running
    st_pending_tasks: Number of tasks currently pending
    st_eta: Estimated Time of Arrival of the currently running tasks
            (in seconds). Note that, internally, the daemon estimates
            how long a task will take by keeping track of the average
            namespace-to-namespace bandwidth from previous requests.
            This means that this field may contain NaN if no tasks
            have yet completed, since the daemon will not be able to
            estimate the E.T.A.

- Changed the format of the global service configuration file to a
YAML-like format
- Urd daemon is now able to read its configuration from the
newly-formatted configuration file.
- Companion libraries linorns and libnornsctl are now capable of
reading their configuration from the global service configuration file.
- Libraries now encapsulate their state in a context structure shared
by all threads rather than in globals.
- Tests are only built now if the --enable-tests option is passed
when invoking configure
- Tests updated to patch the libraries on the fly in order to use
case-by-case configuration files.
- Tests now only cleanup their internal data if the test succeeds.

- Transferors now have a clearly defined API for argument validation
and transfer execution, rather than relying on free functions.
- Added tests for memory to local file transfers.

- Add transferor_registry class to manage resource transfer functions
- First implementation of 'local path' to 'local path' transferor using
  fadvice+fallocate+sendfile.
- Add stubs for other converters
- Move dispatch_table from api to common since it's now also used by
  transferor_registry.
- Add get() function to dispatch_table class so that stored functors
  can be retrieved.
- Generic implementation of task::operator()() to transfer resources and
  resource elements between different backends. When constructed, the
  task is provided with the input and ouput resources as well as a
  'transferor' functor that is invoked upon them to perform the actual
  transfer.
- Add new_resource() and get_resource() methods to backends to promote
  resource_info instances to fully qualified resources.
- Implementation of new_resource() and get_resource() for a POSIX
  filesystem. Stubs for other backends.
- New convenience library that aggregates all symbols for resources
- Add name() and is_collection() methods to class resource_info.
- Provide implementation for boost::filesystem::relative() in utils.cpp
  if boost version <= 1.6.0
- Remove resource::buffer, no longer needed (superseeded by transferors).
- Remove resource::stream, no longer needed (superseeded by transferors).
- Remove functions resource::info(), resource::backend(), and
  resource::set_backend().
- Reorganize resources build hierarchy
- Remove self-registration of backends
- Update tests
- Cleanup unused code

- task_manager is now a full-fledged class rather than
inheriting from std::unordered_map
- changes to handlers to reflect the new interface
in task_manager
- new dry_run option for enabling "fake task mode"
- in libraries, log errors to stderr only if env var
NORNS_DBG_LOG_TO_STDERR is defined.

Added support for the API listener to receive requests from
different endpoints: local sockets for user/control requests and
remote sockets for requests from other daemons. Signals are also
managed by the API listener.

Rename handlers:

urd::create_task      -> urd::iotask_create_handler
urd::check_on_task    -> urd::iotask_status_handler
urd::ping_request     -> urd::ping_handler
urd::register_job     -> urd::job_register_handler
urd::update_job       -> urd::job_update_handler
urd::remove_job       -> urd::job_remove_handler
urd::add_process      -> urd::process_add_handler
urd::remove_process   -> urd::process_remove_handler
urd::register_backend -> urd::backend_register_handler
urd::update_backend   -> urd::backend_update_handler
urd::remove_backend   -> urd::backend_remove_handler
urd::unknown_request  -> urd::unknown_request_handler

Better abstraction and internal management of data
resources and storage backends

Bump spdlog to 0.13.0
Bump catch to 2.0