Port HFileBlockIndex improvement from HBASE-5987

Excerpt from HBASE-5987:
First, we propose to lookahead for one more block index so that the HFileScanner would know the start key value of next data block. So if the target key value for the scan(reSeekTo) is "smaller" than that start kv of next data block, it means the target key value has a very high possibility in the current data block (if not in current data block, then the start kv of next data block should be returned. Indexing on the start key has some defects here) and it shall NOT query the HFileBlockIndex in this case. On the contrary, if the target key value is "bigger", then it shall query the HFileBlockIndex. This improvement shall help to reduce the hotness of HFileBlockIndex and avoid some unnecessary IdLock Contention or Index Block Cache lookup.

This JIRA is to port the fix to HBase trunk, etc.