s3: upload concurrency is now configurable

Please note that if the upload bandwidth between the SFTP client and
SFTPGo is greater than the upload bandwidth between SFTPGo and S3 then
the SFTP client have to wait for the upload of the last parts to S3
after it ends the file upload to SFTPGo, and it may time out.
Keep this in mind if you customize parts size and upload concurrency

httpd/internal_test.go

@@ -369,6 +369,11 @@ func TestCompareUserFsConfig(t *testing.T) {
 		t.Errorf("S3 upload part size does not match")
 	}
 	expected.FsConfig.S3Config.UploadPartSize = 0
+	expected.FsConfig.S3Config.UploadConcurrency = 3
+	err = compareUserFsConfig(expected, actual)
+	if err == nil {
+		t.Errorf("S3 upload concurrency does not match")
+	}
 }
 
 func TestCompareUserGCSConfig(t *testing.T) {