fix: accept upload-id-marker together with key-marker in ListMultipartUploads

The posix and azure backends dropped every upload with a key equal to
the key-marker before handing the list to the lister, which then looked
for the upload-id-marker under the next key. An upload-id-marker that
belongs to the key-marker could never be found, so the request failed
with "Invalid uploadId marker", including when a client paged with the
NextKeyMarker and NextUploadIdMarker the gateway returned.

Keep the uploads of the key-marker itself when an upload-id-marker is
set, so the listing continues right after the given upload, as S3 does.
Enable the REST test that covers it.

Fixes #2149

tests/integration/group-tests.go

@@ -546,6 +546,7 @@ func TestListMultipartUploads(ts *TestState) {
 	ts.Run(ListMultipartUploads_ignore_upload_id_marker)
 	ts.Run(ListMultipartUploads_invalid_uploadId_marker)
 	ts.Run(ListMultipartUploads_keyMarker_not_from_list)
+	ts.Run(ListMultipartUploads_upload_id_marker_with_key_marker)
 	ts.Run(ListMultipartUploads_delimiter_truncated)
 	ts.Run(ListMultipartUploads_prefix)
 	ts.Run(ListMultipartUploads_both_delimiter_and_prefix)
@@ -3254,6 +3255,7 @@ func GetIntTests() IntTests {
 		"ListMultipartUploads_ignore_upload_id_marker":                                     ListMultipartUploads_ignore_upload_id_marker,
 		"ListMultipartUploads_invalid_uploadId_marker":                                     ListMultipartUploads_invalid_uploadId_marker,
 		"ListMultipartUploads_keyMarker_not_from_list":                                     ListMultipartUploads_keyMarker_not_from_list,
+		"ListMultipartUploads_upload_id_marker_with_key_marker":                            ListMultipartUploads_upload_id_marker_with_key_marker,
 		"ListMultipartUploads_delimiter_truncated":                                         ListMultipartUploads_delimiter_truncated,
 		"ListMultipartUploads_prefix":                                                      ListMultipartUploads_prefix,
 		"ListMultipartUploads_both_delimiter_and_prefix":                                   ListMultipartUploads_both_delimiter_and_prefix,