Feature/oracle spread

[dokwon]

Overview

This PR is intended to reduce odds of exploitation in the oracle / swap mechanism. #133 covers the overview of the relevant discussion.

This PR is WIP, and needs updates on unit tests.

Summary of changes

• Add bidirectional Luna supply change cap. Previously, we placed a cap on Luna inflation at 1% daily. We now enforce a Luna inflation/deflation cap change at 0.5%. This prevents large divesting attacks, as well as reducing odds of a consensus attack in case the oracle fails and a malicious user is able to get access to a large amount of Luna tokens.
• Add bidirectional Luna swap spreads: For trades involving Luna, we now charge a 2-10% swap spread, which gets larger as luna supply change trends to its daily change cap on either side. We err on the side of caution to reduce the odds of an attack in the early days of the network.
• Change oracle reward scheme: Previously, oracle ballot winners were rewarded on a monthly basis when the treasury would be settled with 10% of seigniorage during the time period. We are now using the Luna swap spreads that we charge from Luna <> Terra swaps to reward oracle voters in the previous vote period (1 minute). TODO: figure out a way to smooth out oracle returns as there may be heavy swap volume in particular periods
• Swaps halt immediately after an illiquid oracle vote: Previously we've persisted the swap rate of a currency in the store for 10 illiquid votes (drop threshold logic) in order to provided continuity during choppy oracle votes. Now, we stop swaps for currencies after any illiquid vote to prevent a misaligned pricing from providing undue arbitrage opportunities.

Miscellaneous refactor points

• MiningRewardBurden parameter in the treasury has now changed from 20% to 90%. This is to reflect the Terra pre-mine in stability operations, and we no longer need to place a heavy reliance on the budget module
• Budget claim rewards should now be moved from the treasury module to the budget module (WIP)

[dokwon]

small changes