Update IConsumerRegister.Default.cs to make dispose thread safe

[blashbul]

Add memory barrier to _disposed field to make Dispose call thread safe

Description:

When doing Integration testing with WebApplicationFactory Class using xunit, tests tear down dispose twice.
One by CancelationToken, with stop method. This first call make CancelationTokenSource disposed, and every thing is fine.
After that WebApplicationFactory is then disposed, and also call ConsumerRegister.Dispose.
Due to race condition, Pulse method is called a second time and try to dispose already disposed CancellationTokenSource.

It seems to be related to dotnet/aspnetcore#40271

Changes:

• DotNetCore.CAP.Internal.ConsumerRegister.Dispose have been made thread safe

Affected components:

• DotNetCore.CAP.Internal.ConsumerRegister

How to test:

Add an Integration Test project with a WebApplicationFactory, run in Debug mode, exception is logged in Debug output.
Depending your host setup (old fashioned with program and startup class), exception causes test failure with error :
Message:
[Test Collection Cleanup Failure (Data Integration Tests Collection)]: System.ObjectDisposedException : The CancellationTokenSource has been disposed.

  Stack Trace:

CancellationTokenSource.Cancel()
ConsumerRegister.Pulse() line 109
ConsumerRegister.Dispose() line 96
ServiceProviderEngineScope.DisposeAsync()
--- End of stack trace from previous location ---
Host.g__DisposeAsync|16_0(Object o)
Host.DisposeAsync()
Host.Dispose()
WebApplicationFactory1.DisposeAsync() WebApplicationFactory1.Dispose(Boolean disposing)
WebApplicationFactory`1.Dispose()

Checklist:

• I have tested my changes locally
• My changes follow the project's code style guidelines

[yang-xiaodong]

Looks good, thanks for your PR!