Simplify connection recovery

[bollhals]

Proposed Changes

Simplifies how to recover a connection by

• getting rid of the channel for the recovery commands => replaced by recovery method
• only allocates when a recovery is required => reducing overall startup allocations
• splitting recovery in its own partial class cs file for grouping purpose

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

I kind of like the split to partial classes for grouping for cases where we can't break up the implementation (big classes like model or connection). If this is not what we want, I can copy them back.

[michaelklishin]

Thank you!