CosmosStore: Add raw event counts to `eqx stats`

`eqx stats cosmos` determines the event counts by assuming the `n` value of the `Tip` (`-1`) document matches then number of events stored in the other batch documents within any given stream.

This assumption is broken when pruning is in effect (see #226); the solution is to sum `n`-`i` for all documents.

It also makes it harder to demonstrate the degree to which pruning has had an effect in a given container.

Given this is a much more expensive query than the current one, we should consider:
- it should be added as a separate sub-option of the `stats` command
- it should probably not run in parallel with other queries ?