Complete specification for generalised PT composition

obfs-flash (both client and server) already composes two PTs together. We have [ticket:9744] to generalise one aspect of the server component, but there are some things missing for full generalisation; we also need to do this for the client side.

I have some notes here on the abstract design goals we want to achieve: ​https://github.com/infinity0/tor-notes/blob/master/pt-compose.rst

We might also want to do [ticket:7153].