mempool: switch fee estimation to use vsize as a base

In this commit, we modify the fee estimation to use vsize as a base
rather than size. A recent commit landed to track the fee rate using
vsize in the mempool, and also correct some incorrect unit math. This is
a follow up to that commit to ensure that fee estimation is uniform
throughout. To do so, we'll use the KeePerKB directly from the mempool's
TxDesc.

mempool/estimatefee.go

@@ -208,11 +208,11 @@ func (ef *FeeEstimator) ObserveTransaction(t *TxDesc) {
 
 	hash := *t.Tx.Hash()
 	if _, ok := ef.observed[hash]; !ok {
-		size := uint32(t.Tx.MsgTx().SerializeSize())
-
 		ef.observed[hash] = &observedTransaction{
-			hash:     hash,
-			feeRate:  NewSatoshiPerByte(btcutil.Amount(t.Fee), size),
+			hash: hash,
+			// We'll map the fee per KB back into fee per byte by
+			// diving by 1000.
+			feeRate:  SatoshiPerByte(t.FeePerKB / 1000),
 			observed: t.Height,
 			mined:    mining.UnminedHeight,
 		}