AllowConcurrentExecution on IAsyncRelayCommand should default to false

[michael-hawker]

Talking about the new features for 8.0 more, we feel like the majority of applications aren't going to want their async commands to be fired multiple times. For instance, users may click buttons multiple times in an application, but the application probably only expects to run the command for that button a single time.

We feel like for 8.0 we should make a breaking change that all async commands only allow a single execution, and the new AllowConcurrentExecution property/option can be opt-ed in to change this behavior in the constructor of the command or the source generator attribute.