Add network fee monitoring and dynamic fee values

[Reidmcc]

Describe the solution you'd like I'd like Rockfish to know about recent network fee stats to help its transactions go through more quickly. This could potentially improve payment success rates.

Specs:

• pull number of transactions from streamed ledger
• query that number of transactions from transactions endpoint
• calculate fee metrics, e.g. max, mean, mode
• use those data to modify the submitted fee for the next transaction
• add configs to support this, e.g. max fee allowed

If a dedicated Horizon fee stats endpoint becomes available, look into that.

References

• https://github.com/stellar/kelp/issues/108
• https://github.com/stellar/js-stellar-sdk/issues/233