MB-8552: Add support for datatype field

Linking ep_engine change:
http://review.couchbase.org/#/c/31046/

Change-Id: I3d67a79fa685f584db198a47b4758c56d1d9fbfb
Reviewed-on: http://review.couchbase.org/30974
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>