cmux: Remove round robin code and make EWMA mandatory
Since 0.2.4.x stable, Tor has been using EWMA circuit policy which is enabled by the consensus param:
circuitmux.c code still does quite a bit to maintain the round robin circuit policy that was used prior to EWMA. It hasn't been used since 024 (except in Chutney, see legacy/trac#25265 (moved)).
A lot of that code is called for every cell tor receives so it is part of our fast path. Removing this round robin code would help in performance and remove complexity from the code.
However, that round robin functionality is used as a fallback if the EWMA policy was either not set or couldn't pick an active circuit (I'm not sure that is possible if we have active circuits).
Thus, the side effect of removing this code is that we'll now enforce a cmux to have a policy set and make the
pick_active_circuit() callback mandatory. That is OK because we've been enforcing that since 024.
It would also probably mean that we need to put in a default value in tor code for
CircuitPriorityHalflifeMsec so that if the consensus doesn't specify it, we fallback to a usable value instead of being able to disable EWMA.
This piece of work will help out with improving performance of the cmux subsystem with legacy/trac#25152 (moved) and make sure we stick to only the required code for this fast path