OAK-1779: Stale cache after MongoMK GC

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/branches/1.0@1590989 13f79535-47bb-0310-9956-ffa450edef68
OAK-1779: Stale cache after MongoMK GC

Invalidate the docChildrenCache when we detect it is stale

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/branches/1.0@1590990 13f79535-47bb-0310-9956-ffa450edef68