feat(perp): Add a leverage for each vpool as a parameter

[matthiasmatt]

Currently we have a maintenance margin ratio parameter that exists for every vpool and #776 is ensuring that we open positions with enough margin.

We probably want to also add a max leverage per vpool to limit the amount of leverage taken when we open position.

For example a pool can have a maximum leverage of 10, with a maintenance margin ratio of 6.5%. If someone open a position with 10 leverage, they have a margin ratio of 10%, and have 4% of wiggle room before getting liquidated.

This would avoid users opening positions very close to the liquidation zone.