MB-12818: start dir_size in ns_couchdb

Stops dir_size:get using the get_slow code.
Improves performance of couchdb beam process.

Change-Id: I1e838bd20c0d0bacce3477af24458c528c55dca3
Reviewed-on: http://review.couchbase.org/43827
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>