MB-10962 Don't snooze the TAP connection notifier unnecessarily.

There is a race in pausing and resuming the TAP connection
notifier upon receiving a new mutation. To address this issue,
This change adds a flag that indicates if there is a pending
notification event for any TAP connections.

Change-Id: Ide00d835ef3d407087c7313f2346394b7f37c630
Reviewed-on: http://review.couchbase.org/36361
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>