Add more fields to channel acceptor response so that users can have more
fine grained control over their incoming channels. With our chained
acceptor, it is possible that we get inconsistent responses from
multiple chained acceptors. We create a conjugate repsponse from all the
set fields in our various responses, but fail if we get different, non-
zero responses from our various acceptors. Separate merge functions are
used per type so that we avoid unexpected outcomes comparing interfaces
(panic on comparing types that aren't comparable), with casting used
where applicable to avoid code duplication.
This commit adds an optional error message to the channel acceptor's
reponse to allow operators to inform (or insult) unsuccessful channel
initiators as to the reason for their rejection.
This field is added in addition to the existing accept field to maintain
backwards compatibity. If we were to deprecate accept and interpret a
non-nil error as rejecting the channel, then received a response with
accept=false and a nil error, the server cannot tell whether this is a
legacy rejection or new mesage type acceptance (due to nil error),
so we keep both fields.
