Send failures could lock transport into single concurrency mode

[afprtclr]

A port of Particular/NServiceBus.Transport.AzureServiceBus#1066

Serializing the arm/disarm actions is not enough. The state transition preceding the arm/disarm action must be captured in the lock as well, otherwise the state transitions could happen properly but then the arm/disarm actions could be executed by the processor in the wrong order, leaving the logic of the circuit breaker itself intact but the consequences of the arm/disarm actions in an incorrect state.

I also added xdoc as a way to document that the arm/disarm actions are called within a lock, and to maybe pave the way for this to be used as a shared component.