OAK-1793: MongoMK GC removes documents with data still in use

Add currently ignored test

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1592487 13f79535-47bb-0310-9956-ffa450edef68
OAK-1685 BlobStore: improve code readability

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1592488 13f79535-47bb-0310-9956-ffa450edef68