[Fix] mqbs::FileStore: Do not remove purge record even if app is deleted

[kaikulimu]

We had a journal file inconsistency bug if a replica node contains a purge appId record preceding a removal record for that appId, and bounces before rollover. Primary, who rolled over fine, would write that purge record to the new file. Replica, who bounced, would not keep this record in memory during recovery, and thus would not write the record to the new file during rollover. This is because rollover logic decides the set of records to roll over based on the in-memory data structure d_records. This caused journal file inconsistency between primary and replica, where replica's file offset would be behind due to missing certain purge appId records.

The fix is to not skip purge record even if app is deleted, during recovery.