There's no need to use permanent storage for rate limiting information;
try to keep it in memory if caching is enabled.

From experiments with our live setup, this reduces the number of
INSERT/DELETE operations per second from 15 to almost 0. Disk writes on
the server hosting the AUR are reduced by 90% (from ~3MB/s to ~300kB/s).

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>