MINOR: pool: only use opportunistic versions of the swrate_add() functions

MINOR: pool: only use opportunistic versions of the swrate_add() functions

We don't need to know very accurately how much RAM is needed in a pool,
however we must not spend time competing with other threads trying to be
the one with the most accurate value. Let's use the "_opportunistic"
variants of swrate_add() which will simply cause some updates to be
dropped in case of thread contention. This should significantly improve
the situation when dealing with many threads and small per-thread caches.

Performance gains of up to 1-2% were observed on 48-thread systems thanks
to this alone.