compressor: add `CompressKeys|CompressVals` flags to file-header

- use 1-st byte of file to store version. now there is 0, we can assume it's v0.
- increment version of file and store there 1 (v1)
- if file has v0: fallback to existing logic
- if file has v1: reserve 1 byte for feature-flags bitmask
- feature-flags:
```
const (
None = 0b0
CompressKeys = 0b1
CompressVals = 0b10
)
```
let's don't add any logic on this flags now. it's preparation for future experiments.