Add gas price API

[HDauven]

Summary

Provide an API for consumers to request the current gas price. This should not focus on the optimal strategy yet.

Possible solution design or implementation

Take the average gas price of all transactions in the mempool, and the average of the last block, and the highest transaction.

The computational cost of recomputing this value is something that should be taken into account. Indexing the gas price for a transaction must be done.