Cleans up https://github.com/iceiix/steven/pull/57 1.8.9 (47) multiprotocol support
which added too much code duplication, Metadata19 vs Metadata18, and
different packets for each, the only difference being how it was parsed.
Instead, switch back to using only one Metadata implementation, but with
parsing conditionalized on a new global mutable: SUPPORTED_PROTOCOL_VERSION.
Accessing global mutable state is unsafe but it is only set when
initializing the connection, and only read when deep enough in the code
where it is not feasible to pass otherwise. More elegant, worth it.
e28946b691