Constant MEMPOOL_TX_FEE_REQUIREMENT_CAP

pub const MEMPOOL_TX_FEE_REQUIREMENT_CAP: usize = 1000;

The fee cap for MIN_MEMPOOL_TX_FEE_RATE minimum required mempool fees.

Based on LEGACY_DEFAULT_FEE in zcashd: https://github.com/zcash/zcash/blob/9e856cfc5b81aa2607a16a23ff5584ea10014de6/src/amount.h#L35-L36

This is a usize to simplify transaction size-based calculation code.