Make congestion controller aware of all paths

[matheus23]

Currently, each path has its own congestion control - in a weird way.
If I remember the pieces correctly:

• Sending packets off-path for each path isn't accounted for in congestion control at all (? this is likely to be wrong, just remembering)
• Congestion control is per-multipath-path, not per network-path.
• If two network paths share the same bottleneck (e.g. some bad Wifi connection), then we don't coordinate these paths in an ideal way.

Pretty sure we can learn from MPTCP here. Here's one RFC to check out for starters: https://datatracker.ietf.org/doc/rfc6356/