Use strconv.FormatInt() directly to convert key to string w/o using expensive fmt.Sprintf().

common/coords.go

@@ -5,7 +5,6 @@
 package common
 
 import (
-	"fmt"
 	"strconv"
 )
 
@@ -16,5 +15,5 @@ func DecodePosFromBytes(key []byte) (pos int64, err error) {
 
 // Encode a block pos to byte slice.
 func EncodePosToBytes(key int64) []byte {
-	return []byte(fmt.Sprintf("%d", key))
+	return []byte(strconv.FormatInt(key, 10))
 }