MB-23767: Notify vb high priority requests in a separate task

MB-23767: Notify vb high priority requests in a separate task

Vbucket high priority requests are made by ns server during rebalance
wherein it expects a notification when the vbucket receives upto a
particular seqno.

We must send this notification in a separate task rather than in
front end thread to avoid deadlocks due to lock inversion.
That is, a front end thread generally makes call down the stack and
acquires locks in the order top to down (in our software stack).
But a notification path is up the software stack and hence must be
done as a separate task.

This commit creates a daemon task per Ephemeral Bucket to notify
vb high priority reuqests. When the task runs it notifies all the
connections to be notified as goes to sleep. When a new connection
is to be notified the task is woken again.

The current deadlock that was hit is captured in MB-23767.

Change-Id: Id114bedb5cd4de8b493ae6885734c3d440c8bea3
Reviewed-on: http://review.couchbase.org/76659
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>