mix: multiple bug fixes and refactorings for both mix or non-mix logics

* move most data structres in the closer places for readability.
* bug fixes for fetch, propagation, and backfilling for mix aggregation.
* fix tests for mix aggregation.
* when fetching from lower archives, if the time range overlapped data in base/1st
  archive, cwhipser would do a dynamic filling (for both mix and classic aggregations).
  this would slow some reads but for good reason.
* add cmd/read.go.
* cmd/dump.go: print date times along side with timestamps.

cmd/read.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strconv"
+	"strings"
+
+	whisper "github.com/go-graphite/go-whisper"
+)
+
+func main() {
+	var body string
+	if len(os.Args) < 2 {
+		fmt.Println("write: write data points to a whisper file.\nwrite file.wsp [1572940800:3,1572940801:5]\n")
+		os.Exit(1)
+	} else if len(os.Args) > 2 {
+		body = os.Args[2]
+	} else {
+		in, err := ioutil.ReadAll(os.Stdin)
+		if err != nil {
+			panic(err)
+		}
+		body = string(in)
+	}
+
+	db, err := whisper.OpenWithOptions(os.Args[1], &whisper.Options{FLock: true})
+	if err != nil {
+		panic(err)
+	}
+
+	if err := db.UpdateMany(parse(body)); err != nil {
+		panic(err)
+	}
+
+	if err := db.Close(); err != nil {
+		panic(err)
+	}
+
+	if db.Extended {
+		fmt.Println("file is extended.")
+	}
+}
+
+func parse(str string) []*whisper.TimeSeriesPoint {
+	var ps []*whisper.TimeSeriesPoint
+	for _, p := range strings.Split(str, ",") {
+		p = strings.TrimSpace(p)
+		if p == "" {
+			continue
+		}
+		pp := strings.Split(p, ":")
+		t, err := strconv.Atoi(pp[0])
+		if err != nil {
+			panic(err)
+		}
+		v, err := strconv.ParseFloat(pp[1], 64)
+		if err != nil {
+			panic(err)
+		}
+		ps = append(ps, &whisper.TimeSeriesPoint{Time: t, Value: v})
+	}
+	return ps
+}