At log splitting time, we can filter out many edits if we have a conservative estimate of what was saved last in each region.
This patch does the following:
1) When a region server flushes a MemStore to HFile, store the last flushed sequence id for the region in a map.
2) Send the map to master it as a part of the region server report.
3) Adds an RPC call in HMasterRegionInterface to allow a region server to query the last last flushed sequence id for a region.
4) Skips any log entry with sequence id lower than last flushed sequence id for the region during log split time.
5) When a region is removed from a region server, removed the the entry for that region from the map, so that it isn't sent during the next report.
This can reduce downtime when a regionserver goes down quite a bit.