Add new unit tests that perform validation for message IDs.  One test verifies
that there are no duplicate message ID assignments, and the other makes sure
that all message IDs defined are registered with the server and have a valid
default message.  This testing is conducted using reflection to examine all
declared fields in the message classes.

This commit also includes a number of updates to the message classes to fix
problems identified by these unit tests.

OpenDS Issue Number:  974