Router - Dispatch Pools - Protections and tests

[huntharo]

Motivations

Carried over tasks from #26 for more complete implementation.

Acceptance Criteria

• Document the feature in the README.md
• Add a diagram showing the feature in typical scenario
• Add optional regex to validate X-Lambda-Name headers against
• Add optional header name and value to validate before allowing usage of the X-Lambda-Name header
• Create tickets for refactoring or consolidating the periodic timer logic in Dispatcher / LambdaInstanceManager / LeastOutstandingQueue
• Add an exponential backoff list for lambdas that were tried but did not exist
• Add cleanup of idle pools