OPENDJ-1205 Remove network layer from External ChangeLog implementation

More code cleanup and encapsulation.

StartECLSessionMsg.java:
Introduced enums for ECLRequestType and Persistent.
Completed javadocs from javadocs coming from ECLServerHandler.

ECLServerHandler.java:
Replaced individual fields by directly storing the StartECLSessionMsg object.
Changed visibility to private in a few places.
Replaced isPersistent() by isNonPersistent() and getSearchPhase() by isInitPhaseDone().

ECLServerWriter.java
Used better encapsulated methods from ECLServerHandler.

ECLSearchOperation.java, SynchronizationMsgTest.java:
Consequence of the changes to StartECLSessionMsg.