Print allocation statistics in pools and arenas

[gavv]

core::SlabPool and core::HeapArena are two classes through which roc performs most allocations.

It would be convenient to see simple allocation statistics from them in logs:

• total currently allocated bytes
• total currently allocated objects
• average bytes per second during period
• average objects per second during period

We can compute and print this statistics, say, with period of one minute.

There is a class core::RateLimiter that allows to check if next period expired. We can do this check e.g. after every allocation.

[gavv]

Unassigning, so that someone could pick this up.