Refactor Sender crate

[Sajjon]

Note

Target branch is not main, it is cyon_tps_ng, please review PR #86
before this.

This PR contains a major refactoring of the sender crate, with code clean up and performance adjustments.

Notable changes:

• Split huge functions into smaller functions and/or methods - by introducing a stateful SenderApp struct
• Refactor out literal values and replace with consts
• Send transactions in parallel instead of sequentially, since we were not using the streamed result of submit_and_watch anyway

[Sajjon]

We should probably do https://github.com/paritytech/polkadot-stps/tree/cyon_rewrite instead