chainfee: fetch new relay fee from bitcoind

This commit adds a fetchMinMempoolFee function to the BitcoindEstimator
that fetches the current min mempool fee from the bitcoind backend. The
commit then also updates the BitcoindEstimator to use a minFeeManager
for it's minFeeManager member and uses the fetchMinMempoolFee function
to initialise this.

lnwallet/chainfee/minfeemanager.go

@@ -5,6 +5,8 @@ import (
 	"time"
 )
 
+const defaultUpdateInterval = 10 * time.Minute
+
 // minFeeManager is used to store and update the minimum fee that is required
 // by a transaction to be accepted to the mempool. The minFeeManager ensures
 // that the backend used to fetch the fee is not queried too regularly.