Finalize the ServerDisconnectEvent target field.

There is no good reason for it to be mutable in the first place, considering that the event instance is not used after the method is called.
Update Netty and enable Epoll as bugs have been fixed.
Final nail in the coffin for xxx cannot be cast to yyy bug.
Ignore unknown entries in plugin description files.

Fixes #955.
Correct toLegacyText the same way toPlainText was previously