Introducing a clean split between the constant connection configuration
parameters, which are now all in ConnectionConfiguration and the dynamic
connection state (e.g. hostAddresses) which are now in
AbstractXMPPConnection.
Also removed all arguments of login() since the username, password,
resource and callback handler need now to be configured via
ConnectionConfiguration.
Also remove documentation/extensions/messageevents.md, as it's already
in documentation/legacy
c81cd34561