Adding System.Threading.Channels for simplified, better performing code.

[stebet]

Proposed Changes

This adds System.Threading.Channels for a much more optimized and easy to use construct for asynchronous consumer/producer queue (like used in the Async Dispatcher and the Recovery queue). After looking at optimizations and code surrounding the AsyncConcurrentQueue this seems like a much more sane approach.

This adds a new dependency on the System.Threading.Channels NuGet package (supported by all platforms).

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

This package has been used in previous PRs (f.ex. #824) and also the PRs that were part of the bigger Pipelines and allocation PRs, but this stands very well on it's own and is perfectly compatible with both 7.0.0 and 6.X. and is a simple change.

[lukebakken]

Thanks!