Extract LRUCache into generic cache type.

Change-Id: I402da4de408a44457daeec7497d2393c5f65ab66
Abstract high watermark cache into standard LRU.

Conflicts:
	storage/metric/memory.go
	storage/metric/tiered.go
	storage/metric/watermark.go

Change-Id: Iab2aedbd8f83dc4ce633421bd4a55990fa026b85