MB-100: consolidate active_tasks stats

Change-Id: I5d664f03f66555c0ec039b7e4f655c98dbdf3bbe
Reviewed-on: http://review.couchbase.org/24416
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>