Relays should publish number of refill intervals where the token bucket went dry

Our extrainfo descriptors have 15-minute intervals where we list the sum of bytes handled in that interval.

Intuitively, it seems that congestion should have to do with how many times we run out of tokens in our token bucket.

We could track the number of refills where our global buckets run dry, and publish the number of such refills in each 15-minute interval.

We probably want to track read bucket and write bucket separately; differences between the two might teach us something about legacy/trac#4682 (moved) as well.