Ethereum setDifficulty not working
**Describe the bug**

this way context.ApplyPendingDifficulty() alway false, NotifyAsync SetDifficulty  will never happen.
https://github.com/oliverw/miningcore/blob/8d4073394637a93e22bd39ef04e0cdc3ac45d531/src/Miningcore/Blockchain/Ethereum/EthereumPool.cs#L247

Because the pending has been set to null here with 
https://github.com/oliverw/miningcore/blob/8d4073394637a93e22bd39ef04e0cdc3ac45d531/src/Miningcore/Blockchain/Ethereum/EthereumPool.cs#L536