OPENDJ-1430 Some changes are missing from the external changelog

Small enhancements / cleanups.


ChangeNumberIndexer.java:
Always updated lastAliveCSNs as the very last thing because it is used to decide whether the medium consistency point can move forward.


AddMsg.java:
In toString(), renamed the incorrectly named "changeNumber" attribute to "csn" + merged V1 and V2 protocol paths.
Extracted method encodeAttributes().
javadoc cleanup.

DeleteMsg.java, ModifyDNMsg.java, ModifyMsg.java:
In toString(), renamed the incorrectly named "changeNumber" attribute to "csn" + merged V1 and V2 protocol paths.