uhub has the ability to disconnect a 'ghost' user, e.g. if a user
reconnects then uhub will kick the existing user if it is still there.
However, this could also be triggered abusively by having two "tabs" or windows
connect at the same time from the same client. The process of connecting a new
client is rather expensive because it triggers a lot of updates on all connections.
With this change, uhub will only attempt to disconnect the existing client if
it is marked with the "flag_choke" flag, which indicates that messages must be
dropped simply because the client is not accepting data fast enough. This will
cause the new connection to fail because the user is already logged in.
This can be further improved by adding a timestamp for when the connection
previously was provable working. Which is possibly a better and more reliable
way of detecting ghost users.
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