Add streaming support to handle large files

[romankor]

There is problem syncronizing large files , since its done all in memory , eventually you ll run out of it Download to memory --> flush to disk

I implemented some kind of a streaming solution:

class StreamingFileTarget(FsTarget):

    from functools import partial

    def write_file(self, name, source, blocksize=DEFAULT_BLOCKSIZE, callback=None):

        def write_stream_data(fp_dst, callback, data):
            fp_dst.write(data)
            if callback:
                callback(data)

        self.check_write(name)

        with open(os.path.join(self.cur_dir, name), "wb") as fp_dst:
            write_stream_partial = partial(write_stream_data, fp_dst, callback)
            source.read_stream(name, write_stream_partial)

class StreamingFtpTarget(FtpTarget):

    def read_stream(self, name, callback):
        self.ftp.retrbinary("RETR %s" % name, callback)

But def _copy_file needs to be alerted

        dest.write_file(file_entry.name, src, callback=__block_written)