asset: Make spv wallet a FeeRater and use NoLocalFeeRate for spv wallets.

[ukane-philemon]

@chappjc from matrix: I have oracles enabled and instead of going with mempool.space 's fastest fee, it uses the stupid high fee from server here's a bug somewhere. Burned over a $100 on a basic send because of this.

We need to take the approach described in the comments(See comment and NoLocalFeeRate) (for SPV wallet use it as the local estimator function, and make the spv wallet a FeeRater). The other approach would be to make btc.(*baseWallet).Send work like Redeem, using targetFeeRateWithFallback instead of feeRateWithFallback but with Send, the feeRate input is not merely a suggestion.
Basically, the external fee rates only get used for SPV (non-FeeRater) wallets in a handful of internal paths, but it's not usable to the consumer.