Add hook/notification support

[jzombie]

The underlying cache-manager should relay "about to evict" and "did evict" back to integration points.

[jzombie]

Also, groups could spill over to other groups if there is enough space (e.g. each group has a 32 GB limit, grouo A has 16 GB stored, group B has 40 GB; algorithm should be smart enough to know to evict from group B if group A needs more space up to its given threshold).