OPENDJ-1231 (CR-2724) Make the Medium Consistency Point support replica heartbeats


Checkpoint commit that:
- adds i18n
- improves comments and javadocs
- improved ServerState class to internally use a lock free implementation


replication.properties:
Added 2 error messages.

ServerState.java:
Made the implementation use ConcurrentMap + removed all synchronized blocks + significantly change the code in update(), removeCSN() and toString().
Removed getMaxCSN() (never used).

ServerStateTest.java:
Added asserts to testRemoveCSN().