Clear TaskFiber message queue on task finish

Task fibers get reused, so their queue must be clear, lest stale
messages be delivered to a different task.

Signed-off-by: Luca Niccoli <l.niccoli@awtech.it>
Merge pull request #934 from lultimouomo/clearMessageQueue

Clear TaskFiber message queue on task finish. Fixes #933.