Fix reconnecting to servers

**Moving `conn.set(null);`**
NetEvents didn't remove any Forwarders from `NetEvents.forwarders`, because`NetEventsPlugin.removeForwarder(Forwarder f)` has always been called with a `null`-argument.

**Adding `remoteServerUUID.set(null);`**
After reconnecting to a server, NetEvents throws a `IllegalStateException` in `Forwarder.java:95` (commited version: `Forwarder.java:96`).