Irmin_pack_unix.LruSourceAn LRU that support memory-bound capacity via configuration key lru_max_memory. Falls back to entry-based capacity via lru_size configuration key, if max memory is not configured.
add t key weight value maps value with weight to key in t.
Note: Irmin_pack.Pack_value.weight.Immediate weights will be checked to see if they exceed an entry weight limit. The current hard-coded entry weight limit is 20kB.