Allow 1 thread creation per X seconds (Thread creation cooldown)

[kaguy4]

Kokonotsuba is unfortunately quite vulnerable to bot spam. Here is how it looks when someone with a bot and infinite IPs floods the board:

As you can see it was all coming from different IPs, about 6~ threads per second. To fix this issue, when someone creates a thread, there should be a cooldown period where noone else can create a new thread. It should be configurable, but around 15-20 seconds would probably be good as the default.