db: Pre-partition repos to verify in authzFilter

Created by: tsenart

This commit pre-partitions the repos to verify in authzFilter.

name             old time/op    new time/op    delta
_authzFilter-16    1.16ms ±12%    1.09ms ±18%  -6.33%  (p=0.020 n=14+15)

name             old alloc/op   new alloc/op   delta
_authzFilter-16     531kB ± 0%     530kB ± 0%  -0.07%  (p=0.000 n=14+15)

name             old allocs/op  new allocs/op  delta
_authzFilter-16      40.0 ± 0%      37.0 ± 0%  -7.50%  (p=0.000 n=15+15)