Improve verification of connection manager

[coot]

• We should analyse how to verify trace ConnectionManagerTrace messsages.
  Some of them might not be desirable in the trace output:
  TrUnexpectedlyMissingConnectionState.
• Verify that we never have let a unidirectional connection reach any of the duplex states: this is done by verification of connection manager state transitions, but we could also verify that whenever we try this a ForbiddenOperation exception is thrown.
• Verify that connection manager methods never return unexpected value, i.e. UnsupportedState.

[coot]

First to things are done in #3479

[coot]

The last one is done in #3309