Update txpool to handle multiple currencies

[palango]

Part of #6

Extend the txpool to be able to handle transaction with different currencies for gas payments.

In the current implementation we added a multi currency heap to improve performance. For me it's unclear if this is worth the additional complexity and increased diff, so we should do some benchmarking around this to get a better understanding.

Tasks

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1. Add a FeeCurrencyContext which includes the exchange rates and gas price minimums #96
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2. Implement the minimum possible version to handle fee currency transaction in the tx pool #95
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[palango]

It might also make sense to postpone this until ethereum-optimism/op-geth#136 has been merged, as that upstream merge includes the split of the txpool into two sub-pools.

This has been done, I'll merge this into our repo.