Fix fee rate calculation bug when a provider is offline.

[ghost]

When a provider is offline the default fee rate of 10 sats/vB was being used. In a high fee environment. this pulls down the average calculated from all providers. e.g. at the moment fees are 89 sats/vB and when a provider is offline, the resultant average fee shown on Bisq is 69 sats/vB:

bisq.price.mining.providers.MempoolFeeRateProvider$First@7015ebef feeRate: 89 
bisq.price.mining.providers.MempoolFeeRateProvider$Second@2a87ba34 feeRate: 89 
bisq.price.mining.providers.MempoolFeeRateProvider$Third@44592c39 feeRate: 89 
bisq.price.mining.providers.MempoolFeeRateProvider$Fourth@5e180aaf feeRate: 10 

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To address this, the offline provider should not pull down the average:

bisq.price.mining.providers.MempoolFeeRateProvider$First@7015ebef feeRate: 88 
bisq.price.mining.providers.MempoolFeeRateProvider$Second@2a87ba34 feeRate: 88 
bisq.price.mining.providers.MempoolFeeRateProvider$Third@44592c39 feeRate: 88 
feeRate is null, provider=bisq.price.mining.providers.MempoolFeeRateProvider$Fourth@5e180aaf 

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The default fee rate should be chosen only when all providers are offline:

feeRate is null, provider=bisq.price.mining.providers.MempoolFeeRateProvider$First@7015ebef  
feeRate is null, provider=bisq.price.mining.providers.MempoolFeeRateProvider$Second@2a87ba34  
feeRate is null, provider=bisq.price.mining.providers.MempoolFeeRateProvider$Third@44592c39  
feeRate is null, provider=bisq.price.mining.providers.MempoolFeeRateProvider$Fourth@5e180aaf  

See: https://github.com/bisq-network/bisq-pricenode/blob/6073a3948e7567359adf9f167c27d65a7e352d9a/src/main/java/bisq/price/mining/FeeRateService.java#L78-L81

[Emzy]

I will test this on my pricenode.

[Emzy]

Tested ACK After restart with this pr 492b04d: For about 20 sec it showed btcTxFee: 10. (far to low) But then it showed btcTxFee: 107 while another price node shows btcTxFee: 58

[ghost]

The default tx fee (10) is shown until there is some data from providers.

https://github.com/bisq-network/bisq-pricenode/blob/6073a3948e7567359adf9f167c27d65a7e352d9a/src/main/java/bisq/price/mining/FeeRateProvider.java#L30