vcl: support pre/post cb before mq wait

vcl: support pre/post cb before mq wait

Allow vls to register cb functions with vcl pre/post mq sleep. These can
be used to drop/reacquire locks prior/after waiting on vcl mq events.

This then allows multi-thread, as opposed to multi-worker, applications
to share sessions between threads without deadlocking, e.g., multiple
threads trying to read/write/close non-blocking sessions. Caveat:
connects still need to be improved.

Type: improvement

Change-Id: I589aa9dfd0553b0fad54f02ed16c3cda9761a83d
Signed-off-by: Florin Coras <[email protected]>
Signed-off-by: Dave Wallace <[email protected]>