Begin PeerConnection cleanup

* Remove send_raw and cleanup_context
* Use PeerConnection.send when we need to send unencrypted
* Add an extra encrypt=true parameter in CryptoPeerConnection.send method
Clean up PeerConnection

* Disentangle PeerConnection from transport.py
* Let create_zmq_socket throw exceptions and handle them in send
* Do not guard zmqstream calls from exceptions as these happen on
  programming errors
* Guard json.loads from exceptions that may be thrown while processing a
  message that came from the network
* Remove cleanup_context
Rename create_zmq_socket to _create_zmq_socket
Simplify updating peer nickname
Don't decide if the recipient uses ipv6 based on exception thrown
Simplify send even more

* Since we already reraise the exceptions thrown by _create_zmq_socket there is
  no need to try to catch exceptions at all
* Only compress the data only if we are going to send
Update tests for PeerConnection to reflect the changes in the cleanup
fix 3
fix 4