Split neubot in neubot-client and neubot-server

[bassosimone]

This is a necessary step to ease maintainability and move forward. We need, for example, to make Neubot compatible with Python3 and update tests. It is difficult to do this with the current code base, which grew too complex while I was doing my doctorate.

This work is currently being carried on by me, @DavideAllavena, and @helloimcarmine in the neubot/neubot-runtime, neubot/neubot-server, neubot/neubot-client repositories.