Property tests for the mempool.

The mempool collects transactions from downstream nodes, makes them available to upstream nodes, and of course provides the pool of transactions that we use when forging blocks.

These tests for the mempool are not model based, but instead check various simple properties and invariants, for instance:

• After adding valid transactions to the mempool, they can be retrieved.
• Adding invalid transactions from the mempool will report them as invalid, and they are not added.
• Transactions cannot be retrieved after they are removed.
• The mempool capacity is not exceeded

NOTE: the test mempool's default capacity is set to a very large value in module Ouroboros.Consensus.Mock.Ledger.Block. This is why the generators do not care about the mempool capacity when generating transactions for a mempool with the NoMempoolCapacityBytesOverride option set.