Support rsyncable compression

[schtritoff]

Hi,

Rsyncable compression support would be a nice feature, it is common in upstream zlib library (gzip / pigz). As far as I know, no other .NET compression library have this feature. Such compressed file is still completely compatible with deflate compression but has other benefits:

• Ability to use rsync synchronization of large files over slow connection
• Ability to make delta changes of compressed files
• Deduplication for backup systems or filesystems supporting it (ReFS, BTRFS, etc)

There is a stackoverflow question about this issue and Mark Adler answered how to achieve this. Doesn't look like big change to ZipFile.cs and/or zlib implementation in this project but requires knowledge about current implementation. Here is the link: https://stackoverflow.com/questions/40764396/rsyncable-compression-library

References (only in C):

• Windows PIGZ port: https://github.com/kjk/pigz
• Upstream PIGZ rsyncable commit: https://github.com/madler/pigz/commit/1f4ddae80814088075482808ccdd1294fddbbc99 (this is initial commit, there are fixes for this code afterwards)