OAK-1063: MutableTree.enter() simplification

Rename enter() and checkExists() to beforeRead() and beforeWrite() to make their use cases more obvious.
Also added javadocs and did some extra cleanup.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1528485 13f79535-47bb-0310-9956-ffa450edef68