Fix support for the TokuMX 'ur' op (notably for findAndModify). Ensure that when we encounter an oplog.refs entry, the river doesn't bomb out on an NPE.

* Fix oplog.refs oid entry handling
* Fix UPDATE_ROW op
* Validate that we got an item before processing it
* Add explicit log message for NPE on namespace fetch
* Fix findAndModify support for TokuMX
Support oplog.refs and multiple op transactions