Using blocking executor is not recommended for application. But it looks
like some use it just because it's the default and are not aware their
should change it despite of documentation and logging message.
Choosing the application threading model is an important step
of an application.
This change deprecates it, in the future we will just make executor
mandatory. This will ensure that application make a choice.
Also this will reduce headache of oslo.messaging developers to
make the driver code working in a sync and async.
And to finish test coverage of blocking executor is 0%...
This rework some tests to remove logging.captureWarnings() that can
catch unwanted warning of other tests. Tests mocks warning instead.
Related-bug: #694728
Change-Id: Ic67164d12e7a9bed76d6e64ca2ced12e3984ff5f
2566be199a