OPENDJ-1637 Upgrade: server fails to start after upgrade (ClassNotFoundException)

Created empty implementations for the comparators serialized in JE databases.
This allows rebuilding the secondary indexes in JE databases.
Added opendj-legacy maven module to this end, and more generally to ease future migrations.


git-svn-id: https://svn.forgerock.org/opendj/trunk@11317 41b1ffd8-f28e-4786-ab96-9950f0a78031
pom.xml: Fixed formatting issues

git-svn-id: https://svn.forgerock.org/opendj/trunk@11320 41b1ffd8-f28e-4786-ab96-9950f0a78031