mcsRebuildEM - add support to calculate HWM for system cat. files.

System cat. files are not compressed for some reason, that means, those files do not have a special header with `lbid`, `colwidth` and so on. We restore extents for this files from binary blob (initial state) and HWM is not calculating for them because `ChunkManager` does not have an API to read not compressed files block by block.
For big data bases - system files will grow as well and HWM will be greater than initial state, this should be supported.