Sometimes to debug some task manager module, we may want to inspect
the whole tree of descendants of some task.
To make it easier, an api call getting a list of statuses of the requested
task and all its descendants in BFS order is added.
Sometimes we may need task status to be nothrow move constructible.
httpd::task_manager_json::task_status does not satisfy this requirement.
retrieve_status returns future<full_task_status> instead of future<task_status>
to provide an intermediate struct with better properties. An argument
is passed by reference to prevent the necessity to copy foreign_ptr.
It is convenient to create many different tasks implementations
representing more and more specific parts of the operation in
a module. Presenting all of them through the api makes it cumbersome
for user to navigate and track, though.
Flag internal is added to task_manager::task::impl so that the tasks
could be filtered before they are sent to user.
The implementation of a task manager api layer. It provides
methods to list the modules registered in task_manager, list
tasks belonging to the given module, abort, wait for or retrieve
a status of the given task.
