[bisq-network/bisq] [WIP] PriceNode: Add support for multiple ExchangeRateProviders (#4315)

[cd2357]

@cbeams thanks for the feedback.

> One question I don't believe you addressed above: what actually happens in the failure mode of the price tolerance being out of range, i.e. over 1% difference? How do users know what went wrong, and how, if at all, can they recover? (...) We should make sure the failure is a graceful as it can be.

Good point, I hadn't specifically considered it, but I will add a test which ensures this edge case fails gracefully.

Technically, any quick and massive price movement of any currency would cause this (for example, a 3% move within 1-2 minutes). Since pricenodes don't poll the exchangs at the exact same times, but perhaps a few seconds apart from each other, in such a case different pricenodes could get considerably different inputs from exchanges and therefore report rates to Bisq clients which may be "out of range" (more than 1% apart across different instances). However, after 1-2 minutes, when all pricenodes get a chance to poll the exchanges again and update their rates, they would then all be at the relatively same level again, reporting rates "within range".

About testing, I'll write out the specific steps of the tests and ask for tester volunteers on keybase.

About rolling out, you're right, I'll coordinate with @wiz to make sure this is done safely (and there's a contingency rollback plan, in case smth goes wrong).

Thanks again for the input. It is a tedious PR, but hopefully the effort will be worth it.

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