MB-5307 Look up replication documents by type=xdc.
MB-5307 Look up replication documents by type=xdc.

Previous code was trying to find suitable fields in the document. And
in case of success the document would be considered replication
document. But all of that is not needed because all the replication
documents have 'type' field which value is equal to 'xdc'.

Change-Id: I09fe871580ede9a20df55a7d0bdf816c26d979a5
Reviewed-on: http://review.couchbase.org/18950
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

pavel-blagodov/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,daverigby/ns_server,vzasade/ns_server,mschoch/ns_server,membase/ns_server,nimishzynga/ns_server,membase/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server,ceejatec/ns_server,membase/ns_server,daverigby/ns_server,ceejatec/ns_server,daverigby/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,vzasade/ns_server,mschoch/ns_server,membase/ns_server,nimishzynga/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,daverigby/ns_server,ceejatec/ns_server,vzasade/ns_server,daverigby/ns_server