s3: fix SetModTime on GLACIER/ARCHIVE objects and implement set/get tier

- Read the storage class for each object
- Implement SetTier/GetTier
- Check the storage class on the **object** before using SetModTime

This updates the fix in 1a2fb52 so that SetModTime works when you are
using objects which have been migrated to GLACIER but you aren't using
GLACIER as a storage class.

Fixes #3522

backend/s3/s3_test.go

@@ -11,8 +11,9 @@ import (
 // TestIntegration runs integration tests against the remote
 func TestIntegration(t *testing.T) {
 	fstests.Run(t, &fstests.Opt{
-		RemoteName: "TestS3:",
-		NilObject:  (*Object)(nil),
+		RemoteName:  "TestS3:",
+		NilObject:   (*Object)(nil),
+		TiersToTest: []string{"STANDARD", "STANDARD_IA"},
 		ChunkedUpload: fstests.ChunkedUploadConfig{
 			MinChunkSize: minChunkSize,
 		},