Extend RepoIndexState syntax to include modifiers

1. ignore revisions (pick first, not last)
2. include releases at given index-timestamp, but include more recent revisions until a second timestamp
3. pin individual packages to specific revision (via sha256, I think we don't want "count")

The 1st and 2nd points are mutulally exclusive, 3rd should be repeated.

Currently we have

```
hackage.haskell.org,2020-03-22T14:34:00Z
```

like syntax, i.e. *RepoName* *comma* *timestamp*.

In #6597 I made configuration with *colon* instead of *comma*, at least for now. Yet, we need a syntax which works for all of above modifications, is not too convoluted and easy to parse, and perfectly also expandable.

One option is to use spaces.