[tor-dev] Slight obfsproxy API change (#10342)

[George Kadianakis]

Hello Philipp and David,

I'm sending you an email because you are the people who are currently
developing obfsproxy modules.

We are thinking of slightly changing the API of obfsproxy, so that it
becomes more friendly to transport authors (or at least we hope so).

Specifically, we are thinking of having the circuit be an element of
the transport class, instead of being passed as an argument in the
receivedDownstream and receivedUpstream methods. I think David and
Leif were also thinking of this change. Furthermore, we are also
thinking of renaming the 'handshake' method to 'circuitConnected'.
(More details can be found in #10342.)

Do you agree with the changes? If yes, I will merge Ximin's patch for
#10342, and you will have to update your transport codebase
appropriately. Please let me know.

Thanks!