When chosen fee provider is down, Bisq should failover to a new one

[ghost]

The Bisq FeeService should automatically select a new provider when the chosen RPC endpoint is down. It does not, simply keeps querying the same one and writing errors to the log. Users are left using stale fee rate (or the default: 50 sat/vB if the server was down at Bisq startup).

PriceFeedService has an automatic failover which chooses a new provider.

PriceFeedService and FeeService could be redesigned to share the same provider and failover mechanism. This would have the added benefit of opening one less connection (currently each Bisq client simultaneously connects to two separate PriceNodes).

[ghost]

@wiz suggestion: add mempool fees to the getAllMarketPrices endpoint. This will enable a single request from Bisq clients to process market prices and mempool fees. Benefits: better network efficiency & more frequent mempool fee updates (every minute instead of every 5 minutes). PR to follow.