Offline message delivery failures

If a message with invalid XML is persisted into the offline message store, message delivery becomes blocked for all subsequent offline messages addressed to the corresponding user. This is a rare condition that has been observed in multiple installations, possibly caused by improper service shutdown, etc.

Improve the exception handling for the OfflineMessgeStore to write ant such invalid messages into the error log, then remove from the database to allow subsequent (valid) messages to be delivered normally.