jail: Handle jail removal in a dedicated thread

Otherwise a deadlock is possible: the system taskqueue thread removes a
prison and calls vnet_destroy(), vnet_vlan_uninit() destroys the if_vlan
cloner, the vlan_clone_destroy() callback calls taskqueue_drain() on the
thread taskqueue.

Fix the problem by introducing a new thread for jail removals.

Ideally, the taskqueue interface would let consumers define queues
without having to map them to threads, as that'd make it possible to
avoid such deadlocks without extra threads; for now, this is the only
solution.

Reviewed by: jamie
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47991