Variable rate, on-demand transaction ingest

[myawnhc]

Current design: Transaction generator runs concurrently with the processing, generating random transactions that are written to the preAuth map. So transaction generation is competing for the same CPU cores that the processing needs.

Preferred design: Pregenerate transactions and store them (database or whatever); have an ingest mechanism that can vary in rate to drive different loads for testing. Rate should be variable to simulate spikes as expected in real-world workloads.

[myawnhc]

This has been mostly completed - transactions are now pre-generated and stored into the database. Merchant and Account info is eagerly loaded at start-up, and the transaction info is then loaded once everything else has completed loading.

The variable rate for the ingest mechanism is not yet done, so leaving this open until I can circle back to that.

[myawnhc]

Implemented, and rate is adjustable through some throttling behavior that somehow got reverted off main - the throttling controls are implemented in jet-engine branch, might merge changes separately or wait until that branch merges. (I think changes are limited to LazyPreAuthLoader and Launcher classes).