Fix response latency calculation

Previously latency calcualted as Response.End - Request.Start
Where both End and Start is a last and first packets

This calcualtion is wrong, because it is total roundtrip

Correct server latency will be Response.End - Request.End

In addition added new `--input-raw-timestamp-type` option
which allows choose more precise packet timestamp source (if available).

http_modifier.go

@@ -2,9 +2,9 @@ package main
 
 import (
 	"bytes"
-	"strings"
-	"hash/fnv"
 	"encoding/base64"
+	"hash/fnv"
+	"strings"
 
 	"github.com/buger/goreplay/proto"
 )