Implement scheduler mechanism to track lists of channels wanting cells or writes; doesn't actually drive the cell flow from it yet
Temporarily disable scheduler_trigger as unused
Fix unused-arguments warnings
Use event_active, not 0-length timeouts. It's idempotent, too.
Uglify scheduler init logic to avoid crash on startup.

Otherwise, when we authority try to do a self-test because of
init-keys, if that self-test can't be launched for whatever reason and
so we close the channel immediately, we crash.

Yes, this a silly way for initialization to work.
Provide generic mechanism for scheduler to query writeable cells on a channel
Actually call channel_flush_some_cells() from the scheduler
Remove no-longer-used channel_tls_t functions
Add global cell/byte counters and per channel byte counters to channel.c
Make 'make check-spaces' not complain about function pointers returning size_t or double
Track total queue size per channel, with overhead estimates, and global queue total
Implement global queue size query in channel.c
Fix return values from channel_flush_some_cells() to correctly count cells directly written by channel_flush_from_first_active_circuit()
Let the new scheduler handle writes
Schedule according to a queue size heuristic
Add cmux support for inter-cmux comparisons
Add inter-cmux comparison support to circuitmux_ewma.c
Sort the scheduler's channel list by cmux comparisons
Fix compiler warning
Fix scheduler assertion in circuitmux/destroy_cell_queue unit test