eliminate allocations from InboundFrame

[bollhals]

Proposed Changes

Eliminates all allocations for InboundFrame by converting it into a readonly struct.

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

[bollhals]

For @stebet test case, drops the ~7 MB allocation (~10%) for InboundFrame.

[michaelklishin]

Backported to 6.x. Thank you!