alterComment issued two engine.Flag calls (Blocked, then Verified) for
every comment, so a listing of N comments triggered up to 2N BoltDB read
transactions even when many comments shared the same author. Find,
FindSince, User and Last all funnel through it.
Add a userFlagCache that memoises blocked/verified results by site and
user for the duration of a single listing, so repeated authors are
looked up once. alterComment keeps its signature for single-comment
callers (Get) by using a fresh cache; the batch paths share one.
Dmitry VerkhoturovandUmputun
a54d2d2756