The previous approach of emitting a severe log message when a
state (descriptor) was unknown was misleading. There are valid cases
where some states are not known, if, for example, a module was
explicitly disabled.
Using Builder.failOnUnknownStates() in unit tests is far cleaner, as
the existence of unknown states is tested in a controlled environment:
one where are states are supposed to be known.
This is a complete redesign of what was previously
XmppNioTcpConnection. The new architecture allows to extend an XMPP
client to server (c2s) connection with new transport bindings and
other extensions.