db: Optimize authzFilter code path

Created by: tsenart

Summary

This PR optimizes the authzFilter code paths.

Benchmark

name             old time/op    new time/op    delta
_authzFilter-16    56.2ms ± 2%     1.0ms ± 1%  -98.14%  (p=0.000 n=15+15)

name             old alloc/op   new alloc/op   delta
_authzFilter-16    55.3MB ± 0%     0.5MB ± 0%  -99.03%  (p=0.000 n=15+14)

name             old allocs/op  new allocs/op  delta
_authzFilter-16     65.8k ± 0%      0.0k ± 0%  -99.94%  (p=0.000 n=15+15)