Problem: memiavl secondary snapshot format not implemented

secondary snapshot is faster to dump then the full one, can support more frequently dump, it's an important part to improve the UX in general.

Start by locating the memiavl snapshot implementation and comparing the existing full snapshot path with the unimplemented secondary snapshot format. Define what secondary snapshots must contain and how frequent dumping should work; done means the format is implemented and supports faster, more frequent dumps.

Written by the indexing model from the issue text.