HBASE-10201 has made flush decisions per Store, but has not done enough work on HLog, so there are two problems:
1. We record minSeqId both in HRegion and FSHLog, which is a duplication.
2. There maybe holes in WAL accounting.
For example, assume family A with sequence id 1 and 3, family B with seqId 2. If we flush family A, we can only record that WAL before sequence id 1 can be removed safely. If we do a replay at this point, sequence id 3 will also be replayed which is unnecessary.