Some improvements on economic security

[akildemir]

• Puts a 5% buffer between the required stake for minted coins and the current stake if the mint is not for a swap. This is to reduce the rejection of mints for swap transactions. Buffer size can be further optimized.
• Restricts the out-of-pool coins that are on serai chain to 20% of in-pool coins if a pool with a certain liquidity exists for the coin. That is to prevent a DoS where a user mints all the allowed sriXYZ for a given stake and in doing so prevents other users to add liquidity or doing swaps since those operations are also mint coins.

For now, the enforcement of the second restriction only applies to direct mints, a situation where a user directly mints sriXYZ by sending coins to its multisig address. But same out-of-pool but in system sriXYZ situation can be also achieved by on-chain swapping to it or removing liquidity from the pool without burning the received sriXYZ.

This pr doesn't apply the same restriction to those situations for now to not prevent someone from just doing a swap or removing liquidity from a pool since those are critical system functions. Also there are extra steps and hence inherent risks to those ways of achieving more sriXYZ in out-of-pool than allowed. Though those disincentives might solely be enough, it still requires further discussion and maybe thinking of other methods such as charging fees for the out-of-pool sriXYZ.