Extend renter settings endpoint for ratelimiting bandwidth

Created by: ChrisSchinnerl

This PR changes quite a few things about the renter's settings endpoint. All fields of the allowance must be optional to change because we don't want to change the allowance every time we want to change a different setting. That means many tests fail and also have to be adjusted. Now SetAllowance will still sanity check the allowance before applying it, but every value that is not set will cause the setting to be unchanged. If that results in a field being not set in still not being set, SetAllowance should return an error. That means the user can't simply call the endpoint without providing parameters and still expect sane values to be set automatically.