Impl of the teacher's testing program integration.

The idea of our implementation is to add a proxy between the backend and the teacher's program since
our backend does not offer a websocket based API.
This proxy's duty is to offer such API and then forward the incoming requests from the tester
program to our backend.
We had to tweak the backend to add a couple new endpoints for the tester and also we had to tweak
the worker to accept range based hash submission (begin charset bound, end charset bound).
This does not break retrocompability and leave the legacy code intact !

TODO;
We should next consider adding new tests to keep the coverage reasonable.