rpc api of ns_couchdb node

contains almost everything (with the exception of compaction_api)
that ns_server node calls on ns_couchdb node

in this commit it still calls ns_server node, because underlining
services are not moved to ns_couchdb node yet

Change-Id: Ica9dfc5cc88561bf939786bfe1fc9085d2f83edd
move all couchdb related services to ns_couchdb node

Change-Id: I1daf53f3e7e754b3b8c3179f6c351c2b9e78f0a1