Added a file manager for the SortedBlockTable named SortedBlockTableFileManager.
It primarily enhances read throughput by allowing SBT file readers to be
re-used.

* Handles opening file readers and writers for SBT files.
* Provides reader/writer lock mechanism for multiple readers and only a single writer.
* Provides multithreaded file locking capability.
* Tracks opened SBT readers so that they can all be disposed when needed.