Update the codebase to use a feerate for asburdly large fees. In order to simplify wallet fee handling, we removed (in D1800) the wallet-specific mintxfee in favor of only using the minimum relay fee, as the codebase used the maximum of these two amounts. Now, the wallet still uses a maximum fee which is a flat amount, rather than a feerate. This means GetMinimumFee must return an Amount, rather than a FeeRate.
Requiring an Amount from GetRequiredFee inhibits further simplification of the wallet fee handling code.
Depends on D1936