Fix for 982 : reorganize synchronization code :

before synchronization code was in 2 packages :
org.opends.server.synchronization and org.opends.server.changelog

It was not clear that the code in the changelog package is part of the synchronization
also some code in both of these packages are actually used by both sides of the synchronization.

To make this better (and as said in the synchronization meeting) the code is now split in 4
packages that are all below : org.opends.server.synchronization
- synchronization.changelog contain the code running on the changelog side
- synchronization.plugin contain the code running on the DS side.
- synchronization.protocol contain the code used by both side to exchange information.
- synchronization.common contain utilities that can be used by any of the first 3 packages.

Hopefully this should make the synchronization code more easy to understand.