Decide what to do with RELAY_PAYLOAD_SIZE_MAX in dropped-cell calculations

At !878 (comment 3192761) , @mikeperry suggests that we should figure out what to do about vanguards and the use of RELAY_PAYLOAD_SIZE_MAX in the functions circuit_{sent,read}_valid_data in circuituse.c:

This might impact how the vanguards addon detects dropped cells; I have to look at the vanguards addon code to be sure. Either way that doesn't impact the change here, and also might not matter anyway, if the onion service ecosystem is able to migrate to arti.

I got to the end and noticed we have a RELAY_PAYLOAD_SIZE_MIN we could use instead.. I still don't know which yet in this case cause I have to check the vanguards source still..

But maybe the answer is we just make a ticket for these dropped cell related ones, to choose MIN vs MAX, so it does not block the MR?

We should follow through on this.