Implement a "proper" connection throttle for netty.

Proper meaning a simple portover from CraftBukkit's implementation.

The last implementation had a few issues:
- For each server ping, the connection was getting throttled.
- ConcurrentHashMap is heavy (More of an opinion but don't judge)

From the last implementation, the connection throttle entry (your IP) wasn't getting removed from the list after a server ping, which is supposed to happen according to the original implementation.