MB-100 Add xdc_rdoc_replication_srv:get_full_replicator_doc.

It returns a replicator document by id. Contrary to
find_all_replication_docs it doesn't strip any fields from the
document body and ensures that the field names are binaries.

Change-Id: I6e7b95332a5d332e50b29550d16d009e37978bc8
Reviewed-on: http://review.couchbase.org/28176
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>