Refactor snapshot selection

[myieye]

Currently the SnapshotWorker is pretty complicated for several reason.
One reason is that we've embedded snapshot sampling quite deeply inside it.

We want to refactor that to something like this:

• Just collect all generated snapshots in a dedicated class (e.g. PendingSnapshots)
• Then determine the mission critical ones (root and latest). It's not entirely obvious where the "mission critical predicate" code should live
• Then use a dedicated SnapshotSampler class to decide what snapshots will actually be persisted (mission-critical + an intermediate sampling)
• Later we'd like to Implement snapshot cleanup #35