CosmosStore: Add raw event counts to `eqx stats`

[bartelink]

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 ?