.travis, build: autodelete old unstable archives (#13867)

This commit adds a build step to travis to auto-delete unstable archives older than
14 days (our regular release schedule) from Azure via ci.go purge.

The commit also pulls in the latest Azure storage code, also switching over from
the old import path (github.com/Azure/azure-sdk-for-go) to the new split one
(github.com/Azure/azure-storage-go).

vendor/github.com/Azure/azure-storage-go/util.go

@@ -0,0 +1,85 @@
+package storage
+
+import (
+	"bytes"
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/xml"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"reflect"
+	"time"
+)
+
+func (c Client) computeHmac256(message string) string {
+	h := hmac.New(sha256.New, c.accountKey)
+	h.Write([]byte(message))
+	return base64.StdEncoding.EncodeToString(h.Sum(nil))
+}
+
+func currentTimeRfc1123Formatted() string {
+	return timeRfc1123Formatted(time.Now().UTC())
+}
+
+func timeRfc1123Formatted(t time.Time) string {
+	return t.Format(http.TimeFormat)
+}
+
+func mergeParams(v1, v2 url.Values) url.Values {
+	out := url.Values{}
+	for k, v := range v1 {
+		out[k] = v
+	}
+	for k, v := range v2 {
+		vals, ok := out[k]
+		if ok {
+			vals = append(vals, v...)
+			out[k] = vals
+		} else {
+			out[k] = v
+		}
+	}
+	return out
+}
+
+func prepareBlockListRequest(blocks []Block) string {
+	s := `<?xml version="1.0" encoding="utf-8"?><BlockList>`
+	for _, v := range blocks {
+		s += fmt.Sprintf("<%s>%s</%s>", v.Status, v.ID, v.Status)
+	}
+	s += `</BlockList>`
+	return s
+}
+
+func xmlUnmarshal(body io.Reader, v interface{}) error {
+	data, err := ioutil.ReadAll(body)
+	if err != nil {
+		return err
+	}
+	return xml.Unmarshal(data, v)
+}
+
+func xmlMarshal(v interface{}) (io.Reader, int, error) {
+	b, err := xml.Marshal(v)
+	if err != nil {
+		return nil, 0, err
+	}
+	return bytes.NewReader(b), len(b), nil
+}
+
+func headersFromStruct(v interface{}) map[string]string {
+	headers := make(map[string]string)
+	value := reflect.ValueOf(v)
+	for i := 0; i < value.NumField(); i++ {
+		key := value.Type().Field(i).Tag.Get("header")
+		val := value.Field(i).String()
+		if key != "" && val != "" {
+			headers[key] = val
+		}
+	}
+	return headers
+}