Significantly reduced Large Rock (and slightly shared memory) RAM requirements

Significantly reduced Large Rock (and slightly shared memory) RAM requirements
by not allocating 40 (and 12) bytes of unused RAM per cache slot.

Rock: Stale code inherited from the Small Rock implementation allocated 40
bytes of unused memory per rock db slot (2.5GB of RAM for a 1TB disk with 16KB
db slots). The current (Large Rock) code stores the same kind of information
(and more) in a different location (Ipc::StoreMap).

Shared memory: Similarly, the Large Rock-based shared memory cache allocated
12 bytes of unused memory per shared memory cache slot (3.8MB of RAM for a
10GB shared RAM cache with 32KB slots).