Add DispatchConsumersAsyncEnabled property on IConnection (#1611)

[lukebakken]

• Name new property DispatchConsumersAsyncEnabled.
• Add a check on BasicConsume for when a regular dispatcher is used, and an async consumer passed.
• Test the new DispatchConsumersAsyncEnabled property.

Follow-up to #1611
Fixes #1610

[lukebakken]

@luizcarlosfaria this API change will be available in version 7. Thanks!

[niemyjski]

This change is kind of confusing looking at the 7.0 breaking changes.

_factory = new ConnectionFactory { Uri = new Uri(options.ConnectionString), AutomaticRecoveryEnabled = true, DispatchConsumersAsync = true }; ``` DispatchConsumersAsync where did this get moved to now? 

[lukebakken]

@niemyjski would you mind starting a new discussion, or asking in this one, so other people can more easily find my response?

#1720