Listener refactor

### Motivation:
It turns out everyone uses `TransportLayer` as a dump for every method or attribute whose proper place is undetermined. As a result, this class is bloated with all sorts of code: managing sockets, listening for requests and handling various pieces of crypto code.

This is the first of a series of attempts to reduce the bloat of `TransportLayer` and factor the code into several smaller and simpler classes, more amenable to testing and maintenance.

The new `Listener` classes mirror the pattern of `PeerSender` and `CryptoPeerSender` but are designed cleanly and follow good practices such as defining all members in constructors. They are also completely independent from `dht.py` and `transport.py`, whereas `Sender` classes still have this problem and should be refactored in the future.

### Changelog:
* Factor classes `PeerListener` and `CryptoPeerListener` out of `TransportLayer` and `CryptoTransportLayer`, respectively.
* As the `Sender` classes get a `dict` that is then encrypted, compressed and sent, the listener classes do the inverse actions and receive, decompress and decrypt the messages, relaying the sent `dict`s to the receiver.
There is only one listener for each transport layer and hence only one context for listening per transport layer. I am not saying that this is correct. It isn't but I decided to make as little changes as possible while moving code around to ensure that no problem is introduced especially in this part of code that handles networking and crypto stuff