There are some bugs missed in task handler:
- wait_for_task does not wait until virtual tasks are done, but
returns the status immediately;
- wait_for_task suffers from use after return;
- get_status_recursively does not set the kind of task essentials.
Fix the aforementioned.
Virtual tasks are supported by get_task_status, abort_task and
wait_task.
Task status returned by get_task_status and wait_task:
- contains task_kind to indicate whether it's virtual (cluster) or
regular (node) task;
- children list apart from task_id contains node address of the task.
Contrary to regular tasks, which are per-operation, virtual tasks
are associated with the whole group of operations. There may be many
operations of each group performed at the same time. Info about each
running operation will be shown to a user through the API.
For virtual tasks, task manager imitates a regular task covering
each operation, but task_manager::tasks aren't actually created
in the memory. Instead, information (e.g. status) about the operation
is retrieved from associated service and passed to a user.
To hide most of the differences from user, task_handler class is created.
Task handler performs appropriate actions depending on task's kind.
However, users need to stay conscious about the kind of task, because:
- get_task_status and wait_task do not unregister virtual tasks;
- time for which a virtual tasks stays in task manager depends
on associated service and tasks' implementation;
- number of virtual task's children shown by get_tasks doesn't have
to be monotonous.
API is modified to use task_handler.
API-specific classes are moved to task_handler.{cc,hh}.
