Implement max_concurrent_streams_per_circuit
Hi! This branch attempts to implement max_concurrent_streams_per_circuit, using approximately the design from #1124 (closed).
There are aspects of this that I don't like (marked with TODOs), and at least one XXXX about two types with the same name. I'd like feedback about how you think I should handle those.
I don't expect to have the first version of this to get merged
Closes #1124 (closed).
Edited by Nick Mathewson