OAK-1645: Route find queries to Mongo secondary in MongoDocumentStore

Merged r1594159-1594164,1594166-1594167,1594169,1594808 from trunk

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/branches/1.0@1603784 13f79535-47bb-0310-9956-ffa450edef68
OAK-1897: Stale documents in MongoDocumentStore cache

Merged r1603155 and r1603401 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/branches/1.0@1603802 13f79535-47bb-0310-9956-ffa450edef68