SessionState.rollbackToPosition should use unsigned arithmetic

SessionState.rollbackToPosition should treat sequence numbers as unsigned values when comparing them.

Normally this isn't an issue, since sequence numbers greater than 2^63 are unheard of in the wild.

This fix is required by the new Elasticsearch connector, which will create synthetic fallback log entries with seqno -1 (max unsigned value), and these entries must be pruned correctly if a rollback occurs.