Reduce donate token gas costs

[naddison36]

Problem

Donating small amounts of DAI back to the mUSD, FRAX and BUSD vaults is very expensive. For example, it cost 3,264,357 gas (70 USD) to donate 63 DAI back to the mUSD, FRAX and BUSD vaults. https://etherscan.io/tx/0xfb0042c2425a27f4705fc76b1810be015c2d4bf43565f5e3c66a1c78255c054a

Normally this will be held back until there is a significant reward but last week we ran through the liquidation cycle for testing purposes. It's also worth considering that if rewards are not streaming then the APY will close to 0.

Proposed solution

The donate function will convert DAI/USDC/USDT to 3Crv using the 3Pool, but will not add liquidity to the Curve Metapool and deposit the Metapool LP token to the Convex pool. This will be delayed until the next mint or deposit.

The 3Crv can sit in the vault but have no impact on the assets per share. This is effectively an unrealised gain like accrued rewards.

The next time a mint or deposit is done, it will check if the vault has a 3Crv balance. This balance will be added to the mint/deposit 3Crv that is added to the Metapool and LP deposited to the Convex pool. The resulting shares will be proportionally split between the minter/depositor and the vault. The existing streaming process will then work to burn the vault shares over the next week.

[naddison36]

This should be done for the FraxBP contracts first on the fraxBP branch