prevent multiple updates running simultaneously

We're the renderer to request for an update multiple times, then it'd
simply start both updates, at the same time, causing all kinds of
issues. This was only really realistically possible to happen if you
were to manually check for updates whilst an update automatically were
to be checked from right before then. As an example, when Viper starts
up, it'll by default check for updates for itself, then Northstar, if
you within that time frame checked for updates manually, it'll run both.

Now however, we check if an existing update is occurring, and do nothing
if so, this is both for Viper and Northstar updates.

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