Improve Search performance by introducing per-tag index hashes

[Difegue]

I don't think search can get faster than it currently is if we keep looking at every file each time.
The most simple solution imo is to build index tables for each namespace: The structure for this already exists in the stats minion job, it's mostly a matter of extending it so that Search can rely on those stat tables as well.

In case the stat tables aren't built (first searches), we'll probably have to fallback on the existing look-in-every-redis-key algorithm.