Major design change

[chjj]

We could possibly switch to all synchronous read calls (this improves performance a lot by taking pressure off the thread pool). To avoid blocking the main thread, the tree could run as a worker process or on its own thread (with node-webworker-threads).

In reality, the urkel tree should just be rewritten in C and bound to. We could batch multiple operations and dispatch everything in the thread pool at once.