Manage connected peers list

[mycognosist]

There are a few related changes introduced in this PR:

• Rename the secret_handshake module to connection
  • This makes more sense, as that module involves TCP connection handling as well as the secret handshake
  • Connection-related types have been moved into this module (with the exception of ConnectionEvent which remains in connection_manager)
  • TcpConnection
  • ConnectionData
• The list of connected peers is now updated in the connection manager
• Error messages are now capitalised (looks neater in the console output)
• Replication outcome reporting has been cleaned up