code: Improve mempool of Starknet test app

[romac]

Our current mempool for the Starknet mock app is quite barebones and could use a few improvements, namely:

• Split out tx generation out of the Mempool actor and into its own actor (MempoolLoad or something)
• After a block is decided, only remove txs that were included in that block
• Allow configuring the MempoolLoad actor to generate load on the mempool in various ways
  • No load
  • Uniform load configured in TPS (transactions per second)
  • Non-uniform load to simulate peaks in tx ingestion, the frequency and amplitude of these peaks should be configurable
• Allow size of generated transactions to be configured (constant, uniformly distributed, other distributions)

[cason]

Another way to produce load is to emulate what we call a "closed-loop client".

Namely, we tell the number of clients we want to emulate. A client submits a tx, wait until it is decided, then submits another txs, possibly after a (configurable, random, small?) sleep.

In other words, the number of emulated clients is the maximum number of outstanding requests in the system at any time.