We poll the fx rate provider every 2.5 minutes (unchanged).
Previously if there was any error during a tick, there was no fx rate
available in the client until the next tick.
Now, instead, we keep the last rates received with a 10 minute expiry.
One potential drawback is that previously there was instant feedback
to the user when e.g. changing proxy settings, and this is no longer
the case. E.g. consider a provider that bans Tor exit nodes. If a user
enables using a Tor proxy in the network settings, the fxrate used to
disappear immediately - but now the cached rate would still be
available.
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