sched: scheduler_compare_channels() function will never pick a channel with no active circuits.

In the schedulers, scheduler_compare_channels() is used to decide which channel is 'best' to write data from. It delegates to circuitmux_compare_muxes(), which delegates to ewma_cmp_cmux().

But ewma_cmp_cmux() will never prefer a cmux with no active circuits on it! So a channel without active circuits will never be picked by the scheduler to flush from a circuit, which is what triggers flushing from its destroy queue. So the channel will stay around forever, never flushing.

To fix this one, we probably have to fix ewma_cmp_cmux() to look at destroy cells too (somehow). And we still need to make sure that the scheduler's position in the heap changes when the data considered by scheduler_compare_channels() changes [*].

[*] I'm not convinced that we're even doing this right with the current scheduler_compare_channels() code. :(