Implemented MultiThreadedStorageEngine

[mrdlm]

The current storage engine is single threaded. It's asynchronous, but there's only a single thread reading from WriteQueue and only a single FileChannel writing to disk.

Let's implement a multi-threaded storage engine which reads from WriteQueue through several workers, has multiple file channels writing to different files parallel-y at the same time.

There'll be a 1-to-1 mapping from queue-reading-threads to write-file-channels.

hypothesis: this will speed up the writes.

there are competing problems; maybe if you try to write parallelly, you lose the speed-bump which comes from batching writes. This can't be reasoned theoretically, we have to run the experiment and see.