implement updateNewerFiles option

[paganaye]

It is quite long to upload many files over and over again. I needed the updateNewerFiles option. I thought of comparing dates but it is error prone as the file might be modified while we are uploading. I went for a I after a bit a thought I believe it is safer to upload the filemap. I also had to store the the locale modification time for each file in the filemap.

{
    "date": "2022-01-01T13:53:27.498Z",
    "user": "pascal",
    "client": "PASCAL-PC",
    "filemap": {
        "/": [
            {
                "name": "test-inside-root.txt",
                "mtime": 1641045207476.4395
            }
        ],
        "folderA": [
            {
                "name": "test-inside-a.txt",
                "mtime": 1640956927631.6794
            }
        ],
        "fo

I am not a javascript specialist. I did the best I could. It works for me. I don't mind if you merge or not or rewrite it totally.