vshield task manager: abort tasks in stop() on termination

This patch kills the manager thread, and aborts active tasks
rather than sending an exception to the manager thread and
have it do the abort on termination.

Unit tests involving vshield task manager might take longer
as a side effect of this patch.

Related-bug: #1282452

Change-Id: I9e9e41ce7e8969a2ea51bfce96b1303125a24308
Merge "vshield task manager: abort tasks in stop() on termination"