Task ttl can be set with task manager test api, which is disabled
in release mode.
Move get_and_update_ttl from task manager test api to task manager
api, so that it can be used in release mode.
Closes#12894
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.
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 task manager api layer. It can be used 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.
