The minimum relay fee is always ensured to be above our fee floor except
in the very first min relay fee query to bitcoind. This commit ensures
that the fee floor is respected in this first query.
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.