Updated Client:connected to be set once our read and write handlers are setup
and added Client:registered to track when registration is complete, which we
used to do via Client:connected.

Also split out everything except sending the QUIT message from
Client.disconnect to Client.stop so we don't try to send a QUIT message when
we're in a failure state.

Connected to a local ergo server using the cli and verified that exited properly via ctrl-c and bad sasl mechs.