MB-7524 Schedule the stats snapshot before purging TAP connections

When the engine shuts down, we schedule the stats snapshot task
and destroy all the TAP connection instances as part of the
shutdown process. As the stats snapshot task needs to persist
TAP stats, it should be scheduled before purging those TAP
connections from memory.

Change-Id: If1e154d68d2b8fa9f9859b9eda7274dad743a07d
Reviewed-on: http://review.couchbase.org/24759
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>