Added unit tests for encoding/decoding changelog state DB entries.


ReplicationDbEnv.java:
Split readChangelogState() into decodeChangelogState() and readWholeState().
Changed buildServerIdKey() into toReplicaEntry().
Combined buildGenIdKey() and buildGenIdData() into toGenIdEntry().
Made some methods package private for testing.
Made some methods protected for easing unit testing.
Extracted openJEEnvironment() from ctor.
Renamed getOrAddDb() to getOrAddReplicationDB().

ReplicationDbEnvTest.java: ADDED

ReplicationDB.java:
Consequence of the change to ReplicationDbEnv.