[0.6] server/btc: use external fee rates

[buck54321]

The estimatesmartfee RPC is not very responsive to changing conditions and often gives fee rates that are too high. Try to use fee rates from mempool.space or btc.com before falling back estimatesmartfee. Retrieve rates every five minutes, preferring mempool.space. Fall back if rates are stale by 10 minutes.

Some profiling of fee rate sources was done separately. Ask on Element for results if interested.