Emit CIRC_BW event early for dropped cells

While reviewing the dropmark paper (https://petsymposium.org/2018/files/papers/issue2/popets-2018-0011.pdf) and asking the authors to test it against vanguards's dropped cell detection, I realized that since we only normally emit CIRC_BW events once per second, the attack may still have enough time succeed against vanguards.

The fix is to emit the CIRC_BW event as soon as we get a relay cell that does not cause us to update our delivered or overhead byte counts. It's pretty simply to do this. Patch incoming.