Audit Fix 3: Accumlate notes

[ZenGround0]

When settling a queue with multiple rate changes only the last note is kept. We should do something like in finalization passing the note through and concatenating.

I am a bit worried about memory / ret value / processing costs adding up quickly. Maybe we can concat only if strings don't match? Or perhaps enforce really strict limits on note size?

[rvagg]

Oh, this could get messy. Theoretical max, I think: we can only record one rate change per epoch (even if we have multiple transactions processed in an epoch that rate change), so you could, at max, have as many notes as epochs since last settlement. For FWSS, if you addPieces each epoch and an SP settles every 30 days, then that's 86,400 notes! Mind you, processing that many rate changes probably would consume too much gas so you'd have to settle in smaller chunks, but still, that's a lot of notes for one rail to accumulate in state, or a very large string to accumulate if we concatenate.

[BigLep]

For the record, this came out of #253 but wasn't deemed gritical for GA, which is why it has an MX priority.