mempool/estimatefee: make 1 Kb = 1000 bytes

This commit changes the value of bytesPerKb to 1000 from 1024.
This is done to ensure consistency between the fee estimator
and the mempool, where the feeRate is set to
    fee * 1000 / serializedSize

mempool/estimatefee.go

@@ -45,7 +45,7 @@ const (
 	// it will provide fee estimations.
 	DefaultEstimateFeeMinRegisteredBlocks = 3
 
-	bytePerKb = 1024
+	bytePerKb = 1000
 
 	btcPerSatoshi = 1E-8
 )