Implement mempool transaction storage

[mpguerra]

Motivation

We need to implement a mempool for transactions that have not yet been mined into a block

Specifications

This will be a very basic initial implementation of a mempool that can hold one transaction at a time

• Create a stub mempool that is either just a hashmap or a struct wrapping a hash map, or one or two hash maps, to hold our tx’s, instead of logging them
  • must have methods to insert and remove transactions from the mempool
  • must handle WTX ids

Designs

https://jamboard.google.com/d/1HUhdqx8afJUqgiBdBcjCw0aAfDod9R9KBJeAH24U1Qg/viewer?f=0
We might want to create a new module to hold this in the zebrad crate