Refactor the Ztoc creation

[vkuzniet]

Is your feature request related to a problem? Please describe.
Per #103, the snapshotter's Ztoc creation needs to be adjusted to allow for easy addition of new types of compression algorithms. For more details, please refer to the design doc in #103.
This story requires implementing the following:

1. TocBuilder (please ensure that this type will be a common one for any compression algorithm)
2. Interface ZinfoBuilder with specific implementation for gzip (aka GzipZinfoBuilder)
3. ztoc.Builder, which combines 1 and 2. It will be shared for any compression algorithm. For more details see the design doc from Ztoc should be separated from compression #103.
4. soci.buildSociIndex must be using ztoc.Builder for building Ztoc.

Describe the solution you'd like
The steps to implement are outlined in the above section.

Describe alternatives you've considered
Keep everything as is.

Additional context
#103