OAK-1117 - [MongoMk]Flag document with children

Adding support for marking nodes which have child nodes.
* Added DocumentStore.getIfCached replacing unused isCached
* Added a new boolean attribute `_hasChildNodes` to mark nodes which have child
* Modified Commit logic to mark parents by setting `_hasChildNodes` to true
* Changed assertion in two testcase as with this change the commit root might change
  in some cases

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1536295 13f79535-47bb-0310-9956-ffa450edef68
OAK-1117 - [MongoMk]Flag document with children

Minor updates
* Change the new attribute name to `_children` from `_hasChildren`
* Now the boolean flag would only be added for nodes which have child
  so by default a node doc would not have this flag set

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1536296 13f79535-47bb-0310-9956-ffa450edef68