OF-1515: Migrate Private XML storage to PEP.
Openfire supports XEP-0049, Private XML Storage. However, Openfire can also be used for XEP-0223: Persistent Storage of Private Data via PubSub. When a user previously used more
than one client, where each client would use a different XEP to store private data, the
private data that was stored by one client would not be shared with the other client. This
is undesirable, as such data typically includes data that benefits every client used by the
user.
In this commit, all data that is stored in the Private XML Storage database table will be
migrated to PEP. After a successful migration, the old database table is deleted.
Openfire continues to offer XEP-0049: Private XML Storage functionality. However, whenever
data is obtained or modified through methods defined in that XEP, the PEP data is modified.
