Middleware fixes & refactor

[botchris]

• Pool:
  • Fixed functional options not being applied
  • Improved documentation which better explains how "queues" and "blocking" behaves.
• Retry:
  • added functional options
  • now by default, backoff strategies are used when not specified
  • added OnPublishError option to handle publish errors when retries are exhausted. By default a log is printed.