Added a synchronous write loop for connections.

[michac]

Proposed Changes

Add an option to make the WriteLoop in the SocketFrameHandler run in its own dedicated thread. In this version (6.x) most operations are blocking (CreateModel, QueueDeclare etc...) using BlockingCell. This can create a situation where all the threadpool threads are blocked waiting for a response to a message that can't get sent because there are no threads left for the WriteLoop. See #1354

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue Synchronous operations like CreateModel block their thread waiting for a response to an async Write that they could be blocking. #1354)
• 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

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• 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

If this is a relatively large or complex change, kick off the discussion by
explaining why you chose the solution you did and what alternatives you
considered, etc.

[paulomorgado]

Why should the RabbitMQ client account for application thread management?

The tread pool can be configured to accommodate the needs of the application.

[lukebakken]

Please see #1392