OAK-2267: Expose checkpoints through JMX
Checkpoints MBean for the DocumentMK, Marcel's patch

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1640810 13f79535-47bb-0310-9956-ffa450edef68
OAK-2267: Expose checkpoints through JMX
Factor common parts between implementations into base class

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1640811 13f79535-47bb-0310-9956-ffa450edef68