Nicer API for writing a `Backend`

[utdemir]

Currently the API to write a Backend is quite limited, it forces us to create a new process, and does not look pretty. With a nicer API:

• We can use #5 to get rid of the Python wrapper we're currently using
• We can create a backend which uses threads and an in memory shuffle store, it should be considerably faster than localProcessBackend and localTmpShuffleStore.