irmin-pack: improve UX of the LRU parameters

[samoht]

From https://github.com/mirage/irmin/pull/2050#discussion_r956172914

From a UX perspective, I think it would be nice to change (not in this PR) the configuration to max memory vs max entries since it's much easier to think about how to configure that based on your system. This point came up when I was documenting the configuration keys recently.