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.

mempool/estimatefee.go

@@ -208,7 +208,7 @@ func (ef *FeeEstimator) ObserveTransaction(t *TxDesc) {
 
 	hash := *t.Tx.Hash()
 	if _, ok := ef.observed[hash]; !ok {
-		size := uint32(t.Tx.MsgTx().SerializeSize())
+		size := uint32(GetTxVirtualSize(t.Tx))
 
 		ef.observed[hash] = &observedTransaction{
 			hash:     hash,