forget: redesign current retention policy strategy

While the current `forget` command allows us to specify several options for which snapshots to retain (e.g., `2 monthly`, `1 weekly`, `2 daily`...), it lacks flexibility. For example, it's not possible to specify that we want `2 monthly snapshots` - one from the beginning of the month, and another halfway through the month - while having `3 daily snapshots` that get overwritten each day, without overwriting the "`stable`" monthly ones.

We should reconsider the forget retention policy to better reflect our needs.