Load hashes before Mempool feature initializes

[quantumagi]

https://app.clickup.com/t/1vx67gg

[noescape00]

@quantumagi have you tried to fill mempool, shutdown and start? Did it fix the problem?

[quantumagi]

The purpose of this PR is to fix the null value exception seen in the stack trace accompanying the ClickUp task.

Inspecting the trace revealed that private List<uint256> whitelistedHashes was null when the mempool's InitializeAsync was being called.

By tracing the code I established that the PoAFeature.InitializeAsync, which initializes that value, is only called after the mempool's initialize. This was identified as the root cause of the exception.

Setting the whitelistedHashes value to non-null before the mempool is initialized would 100% avoid the exception. This could be done by setting the value earlier, in the constructor, instead of the InitializeAsync method.

After implementing the fix I performed unit testing by once again stepping through and debugging the code. I confirmed that the value was now non-null when the mempool's InitializeAsync was being called.

@quantumagi have you tried to fill mempool, shutdown and start? Did it fix the problem?

@noescape00, I've left additional testing to the integration testing phase. We won't see this null error again though.