authz: better heuristic of `NextSyncAt` by `PermsSyncer`

Created by: unknwon

We currently reuse the value of UpdatedAt column which sync permissions as fast as we can, it is generally not a problem as long as rate limiter allows.

There is a side effect in current implementation that we have BIGINTSERIAL for user_pending_permissions table's surrogate key and every ON CONFLICT increases its value by 1 silently, but not an immediate concern.