Create common.hpp to access them

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

Also, replace all static_cast<T*>(base.get()) constructs by
utils::static_unique_ptr_cast<T>(std::move(base)).
For example:
    auto request =
        static_cast<api::iotask_create_request*>(base_request.get());
becomes:
    auto request =
        utils::static_unique_ptr_cast<api::iotask_create_request>(
            std::move(base_request));

Initial implementation of a basic I/O task

Better abstraction and internal management of data
resources and storage backends

Bump spdlog to 0.13.0
Bump catch to 2.0