RTCSctpTransportState: state transition clarifications

[aboba]

Fix for Issue #635

[lgrahl]

I believe calling start twice should still raise an InvalidStateError for now. Because, if not, what is supposed to happen? We have discussed this briefly in #625 starting with #625 (comment). The corresponding issue w3c/webrtc-pc#979 indicates that this still needs to be discussed.

[aboba]

@lgrahl I believe that calling start transitions from new to connecting and after that it is not possible to transition back to new. So if we were to throw an InvalidStateError if start is called when RTCSctpTransportState is not new, then I think this would cover the case where start is called more than once.

[lgrahl]

I agree. Your latest commit has made it clear, thanks. 👍