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.

Transfer plugins now receive a new 'context' object that provides access
to relevant data structures from the daemon. This way, they can query
information about the current configuration and status without directly
involving the daemon object.

Fixes #14. Closes #9.

The duration of 'sleep phases' in dry run tasks can now be defined both
in the command line (by passing an integer with the number of usecs to
wait) and in tests (by using the dry_run_duration field in the
fake_daemon_cfg class).

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.

The nornsctl_send_command function now supports a new nornsctl_command_t
command called NORNSCTL_COMMAND_SHUTDOWN, which is accepted and
acknowledged by the server.

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 norns library and service now supports a new type of i/o task called
NORNS_IOTASK_REMOVE that enqueues the deletion of a resource from
a backend. As of right now, only the deletion of NORNS_LOCAL_PATHs is
supported.

This commit defines the NORNS_IOTASK_REMOVE subtype and also implements
the basic mechanism so that a task of this type can be executed.

Added a new 'log_file' option to urd's configuration file that instructs
it to save all log messages to the configured file. As of now, this
option is only honored if 'use_syslog' is set to false.

This commit closes #4.

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.