Add 'counted' list which maintains size

This makes it easier to track the number of items within a list without
maintaining an external counter

In the future, we'd like to consolidate this into a uniform
implementation of list/node types. See CCBC-317

Change-Id: I449c52c3b91e9d13230a5576f71ba4be54cf4c5a
Reviewed-on: http://review.couchbase.org/33110
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>