Vanilla technically allows chat messages with starting slashes now,
Spigot still accepts them as commands, most likely due to being too
lazy to properly differentiate between chat and command intent in
their implementation. This disallows modified clients to send chat
messages with slashes and makes sure chat validation always happens
on the netty event loop, rather than there and possibly being moved
to the main thread, thus having the delayed handling cause a bad
process order of message ids.
