Add OVERHEAD and DELIVERED fields to CIRC_BW events

I want to have information about how much data is actually for applications, so that external controllers can check for circuit side channel attacks.

To do this, I want to add two fields to CIRC_BW events: a PACKAGED field that reports the sum of all rh.length values for all relay cells, and a DELIVERED field that reports the sum of all rh.length for valid RELAY_DATA cells.

That way, controllers can check for large differences between PACKAGED and DELIVERED, to see if lots of data is being sent on a circuit that is getting silently dropped. They can also check for differences between the raw CIRC_BW value and DELIVERED, to see if an adversary is underfilling cells (though this may be far more prone to false positives).