make IncomingCommand a class and simplify code around it

[bollhals]

Proposed Changes

Similar to the frame one, there's also just one Command at a time. So it can be allocated once and updated throughout the lifetime.
This way it can be changed to a class instead of struct, and thus avoids all the complexity of the struct handling especially with the async code.

Slight performance increase as well.

Method	Mean	Error	StdDev	Ratio	Gen0	Allocated	Alloc Ratio
Pr_Publish_Hello_World	275.2 ms	5.50 ms	8.57 ms	1.00	1000.0000	5.85 MB	1.00
Main_Publish_Hello_World	279.8 ms	5.55 ms	11.47 ms	1.00	1000.0000	5.84 MB	1.00

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)
• Performance / simplification

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

I've changed a few things along the way

[lukebakken]

Thank you!