Test for HBASE-9915 (avoid reading index blocks)

In HBASE-9915 we found that for encoded blocks the quick check whether the seek key falls on the current block did not work.

This adds a unittest so we won't regress. The test counts the number of blocks accessed (real data/index block in 0.94 and cache hits in trunk). Without HBASE-9915 we see about 50% more blocks hit in this test.