Ztoc should be separated from compression

[vkuzniet]

Is your feature request related to a problem? Please describe.
Right now the implementation of Ztoc handling is very dependent on Gzip. This makes it really hard to plug in the new compression algorithm, e.g. zstd.

Describe the solution you'd like

1. The design exists to propose how to decouple the Ztoc and compression.
2. Once the design is agreed on, create the tickets to implement the changes.

Describe alternatives you've considered
Keep everything as is. But this tight coupling is hardly extensible.

Additional context
N/A

[vkuzniet]

Design doc: Separation of Ztoc and compression algorithm.pdf