OAK-1209 - Consolidate Mongo client connection creation logic in MongoMK

Refactored the logic to use Mongo URI to create connection. Also merged
MongoNodeStore and MongoMicroKernal service to one. The MongoNodeStore
service would register MK interface if `useMK` config property is set.  Otherwise
 it would default to register NodeStore service. More details in bug notes

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1550394 13f79535-47bb-0310-9956-ffa450edef68