azureblob: implement --use-server-modtime

This patch implements --use-server-modtime for the Azureblob backend.

It does this by not reading the time from the metadata if the global
flag is set.

backend/azureblob/azureblob.go

@@ -1590,7 +1590,9 @@ func (o *Object) setMetadata(metadata map[string]string) {
 		for k, v := range metadata {
 			o.meta[strings.ToLower(k)] = v
 		}
-		if modTime, ok := o.meta[modTimeKey]; ok {
+		// Set o.modTime from metadata if it exists and
+		// UseServerModTime isn't in use.
+		if modTime, ok := o.meta[modTimeKey]; !o.fs.ci.UseServerModTime && ok {
 			when, err := time.Parse(timeFormatIn, modTime)
 			if err != nil {
 				fs.Debugf(o, "Couldn't parse %v = %q: %v", modTimeKey, modTime, err)

docs/content/azureblob.md

@@ -80,7 +80,13 @@ docs](/docs/#fast-list) for more details.
 The modified time is stored as metadata on the object with the `mtime`
 key.  It is stored using RFC3339 Format time with nanosecond
 precision.  The metadata is supplied during directory listings so
-there is no overhead to using it.
+there is no performance overhead to using it.
+
+If you wish to use the Azure standard `LastModified` time stored on
+the object as the modified time, then use the `--use-server-modtime`
+flag. Note that rclone can't set `LastModified`, so using the
+`--update` flag when syncing is recommended if using
+`--use-server-modtime`.
 
 ### Performance