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.
