snazzer-prune confusingly retains more snapshots than expected when only a few snapshots exist

[csirac2]

This bug item is pretty much to address https://github.com/csirac2/snazzer/blob/master/doc/snazzer-prune-candidates.md#bugs-and-limitations "When some datetimes are close together, they mightn't be pruned"

I've considered adding a new retention variable, SNAZZER_RECENTS_TO_KEEP - a linear number of snapshots to retain. But this would still be a "minimum" type policy, just like the other _TO_KEEP variables. Instead, the user-submitted bug report probably desires a SNAZZER_MAX_SNAPSHOTS_TO_KEEP. Which is probably worth implementing, but I should fix the existing confusing behaviour first, because I suspect the desire for this feature is partly driven by the existing surprising behaviour (failure to prune snapshots when there's not many snapshots in the beginning of a deployment).