Improve estimation of memory usage when opening gzipped file

When opening gzipped file (local or by http), the memory usage (uncompressed size) is estimated very roughly: in LoadDocumentTask::prepare() there is hard-coded memUseMB *= 2.5. But if the file is local, we should use last 4 bytes of it to determine real uncompressed size (ISIZE field of gzip format). Even if it is not possible, the compression level may be estimated using some part of the begining of the file.