Previously, the server protocol was detected in the server listing GUI,
and saved for each server in a server_versions.json file. connect_to()
would read this file to get the version to use, an ugly hack/workaround
for threading/intercommunication challenges, and a questionable attempt
at avoiding another round-trip exchange.
Now, the server is re-pinged in connect_to() to get the protocol version
to use. This is a blocking ping so it may somewhat slow down server
connecting, but avoids the race conditions with the server list, and
undesirable intertwining the GUI with the server connection logic.
This hack was present since the original multiprotocol support (#18),
but it was blocking other enhancements, about time to remove it.
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