S3A prefetch - Implement LRU cache for SingleFilePerBlockCache

Currently there is no limit on the size of disk cache. This means we could have a large number of files on files, especially for access patterns that are very random and do not always read the block fully. 

eg:

in.seek(5);

in.read(); 

in.seek(blockSize + 10) // block 0 gets saved to disk as it's not fully read

in.read();

in.seek(2 * blockSize + 10) // block 1 gets saved to disk

.. and so on

The in memory cache is bounded, and by default has a limit of 72MB (9 blocks). When a block is fully read, and a seek is issued it's released here. We can also delete the on disk file for the block here if it exists. 

Also maybe add an upper limit on disk space, and delete the file which stores data of the block furthest from the current block (similar to the in memory cache) when this limit is reached.