Use a LRU list to cleanup the cache.
Avoid a ever-growing cache.
Code cleanup.
Extract cache_entry in its own file.
Update cache statistics for new LRU implementation.
Merge branch 'master' into lru_list
Extract lru_list in its own file.
Change typedef name.
Merge branch 'lru_list'

Conflicts:
	sdd/mem/cache.hh
Simplify ref_counted.
Avoid useless memory allocation when updating a cache entry access date.
Add a shifter predicate for user's functions.
Merge branch 'master' into linear_allocation