Pipes

• Major Rework of internal pipes but no public API changes. We kept running into issues with pipes once returned to the cache would still have outstanding references and cancellations would sometimes cross pipe instance barriers. This PR removed caching and all pipes are now created for each pipe usage. This creates a little memory pressure, but we can re-evaluate this later. Possibly caching the reader and writers instead of the whole NexusDuplexPipe.
• Fixed longstanding issue with cancellation of PipeReader under certain high concurrency circumstances.

Benchmarks

• Created benchmarks for common scenarios.
• Added benchmarks to readme

Misc

• Fixed miscellaneous tests which failed due to concurrent counting.