OIP-3 Coin Ratios on Withdraw Should Exclude Curve

When OUSD asked a curve based strategy for it's balance of a particular coin, the strategy responses with virtual price total number of stables / 3. This is because the actual ratios on curve could under control of an attacker, and the and that the ratios change over time. The curve strategies don't really hold exact specific stablecoins.

Our current system results in emptying out whatever coin we have the least of on withdraws, when we have a funds in curve. This is exactly what we don't want to be doing.

For example, let's say we have 100 million in 3pool, 10 million USDC, and 2 million DAI, and 1 million USDT. Roughly speaking a withdraw request for only 3 million coins is going to empty all the free USDT out.

Off the top of my head, we could either only use the ratios in the default strategies to withdraw from (allowing us to choose to some extent which coins are withdraw, and maximizing the OUSD able to be withdrawn for a given default allocation.)

Or we could just exclude curve based strategies from these calculations.

The issue names no files, tests, or entry points. Start by tracing how Curve strategy balances feed the withdrawal coin-ratio calculation, then compare the two alternatives described: using default-strategy ratios or excluding Curve strategies. Done means the chosen behavior no longer preferentially empties the least-held coin during withdrawals.