LLAP: LowLevelCacheImpl does not match cached ranges properly

LowLevelCacheImpl uses skip list to map start disk range offset to disk ranges. Find for match in the skip list uses cache.subMap() to identify correct disk range. This will not work if the requested disk range offset does not exactly match the cache disk range offset.
For example: If the cache disk range is 0 - 1000 bytes and if the requested disk range is 500 - 1000 there will not be cache hit.

We should probably use floorEntry() to get the greatest entry less than or equal to provided key.