ike_sa: Defer task manager destruction after child destruction

ike_sa: Defer task manager destruction after child destruction

This patch exports the task manager's flush to allow flushing of all
queues with one function call from ike_sa->destroy. It allows the
access of intact children during task destructoin (see git-commit
e44ebdcf) and allows the access of the task manager in
child_state_change hook.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>