Messaging abstraction

Key points:
Any future additional messaging system used to replace IRC will only have create an alternative module to irc_messaging.py to replace the functions start_connection(), send_raw(), send_msg(), receive_single__msg() and msg_receiver(), whose functionality are explained in comments in the file irc_messaging.py (of course there may be some extra details but they will be few).
Almost all references to IRC in the layer above have been excised.
Additionally, a few other functions which are shared between auditor and auditee are now in shared/tlsn_common.py to reduce duplication.