Anti-spam measures

[Minoru]

Inspired by what fediverse.network does to detect spam instances:

1. group instances by "topmost registrable domain", e.g. foo.example.com and bar.example.com get grouped by "example.com";
2. every day, check only one instance of the group. Or, in other words, the check interval for an instance would be number_of_instances_in_a_group*day.

As a result, a spammer that created a million instances won't affect a spider's load at all, as only one of the million instances will be checked every day. It still affects the databse size though, and adds overhead in detecting groups.