track yawning's repo for obfs4
History of OperatorFoundation's fork of obfs4 diverged from upstream some time ago.
this is a fucking mess.
Operator added STUN-aware UDP proxy mode https://github.com/OperatorFoundation/obfs4/commit/53a88b16eb7222a28c71ece1fcf859e85d262025, that we might be interested in, but there's no significant work after 2016.
Yawning is not actively maintaining obfs4 (most of the work there since 2016 is on meek_lite), but there's a fix for the teardown behavior that can be used to break active probe-resistance, as pointed by Frolov's paper https://github.com/Yawning/obfs4/commit/1a6129b66ff3e66c347b54fbae203c1c61d12d74 - I'd say we want this one at least. But I don't like the idea of having to manually maintain yet another repo :O