Schedule Conns / pending ops notifications through thread pool

This change removes a notification thread and schedules its tasks
(i.e., paused connection notification and pending ops
notification tasks) through NON-IO threads in the global
thread pool.

Change-Id: Ia8f20d14cc1878c5a702f34dd0a3f726d26b2190
Reviewed-on: http://review.couchbase.org/30906
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>