um size of any single block. It can have any value > 16 bytes.
    In which case, encoding and decoding buffers do not need to be synchronized.
    Actually, data can be produced by any source compliant with LZ4 format specification, and respecting maxBlockSize.
  - Synchronized mode :
    Decompression buffer size is _exactly_ the same as compression buffer size,
    and follows exactly same update rule (block boundaries at same positions),
    and decoding function is provided with exact decompressed size