MB-22695: Add stats for datatypes

MB-22695: Add stats for datatypes

This commit adds counts for datatypes of documents whose Storedvalue is
resident in memory.

Adds the datatype stats to the 'all' engine stat call. This means that
they will be added to the ETS tables in ns_server.

Introduces a regression test for MB-23892.

This patch was originally e4606e8 and was reverted by 6501c72 due to
MB-23892.

Change-Id: Ic0961af1a3c18362004369db0ff0fa7a9eeba22c
Reviewed-on: http://review.couchbase.org/76617
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>