Release GIL during lcb_wait

Fixes potential issues if a thread is blocked on a long-running IO
operation

Change-Id: I8b6991c4d88b6ec04864e43ad3721d21d21be10d
Reviewed-on: http://review.couchbase.org/26094
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>