increase mandatory ringsize in next hardfork #2

Many, many more rings! My vote is for static ringsize 69.

Discussions: https://github.com/monero-project/monero/issues/1673, https://github.com/monero-project/monero/issues/3035, https://github.com/monero-project/monero/issues/3069

@SamsungGalaxyPlayer's Excel document with the data and calculations

Bumping the ringsize to 69 increases the pre-bulletproof transaction size by about 63% and increases the verification time by about 230%.

Take the data on churning with salt. It's an ongoing area of research.

@SamsungGalaxyPlayer we are thinking about increasing the ringsize when we fork to v8. I've taken a look at @knaccc's javascript code. Is there an estimate of what range proof sizes and EC mult counts for bulletproof verifications might be?

@wowario I'm looking at the bulletproof data now. I'll check back here when I have more data.