Change AllowRequest logic to follow Netflix hystrix implementation

[sidai]

Problem to resolve:

The current AllowRequest will perform a CAS on the openedOrLastTestedTime. Given a scenario where we want to perform a AllowRequest to determine if to use an external service before invoke the hystrix.Do/Go function for actual execution, once the circuit is open, the AllowRequest function in hystrix.Do/Go will always return ErrCircuitOpen since the openedOrLastTestedTime just been updated by the first AllowRequest call.

This can cause the circuit to always open without having chance to measure whether the external service has recovered.

Observation:

The Netflix circuit breaker actually provide allowRequest and attemptExecution function to resolve this case. Where the allowRequest won't change circuit status and is used externally to pre-check if a service call should be permitted before the actual execution. For attemptExecution, it is used internally right before the execution to ensure only the first request is let through after each sleep window.

Fix:

This MR follow a similar implementation to update the existing AllowRequest function which could be used externally and add a attemptExecution to be used internally right before the actually execution in hystrix.Do/Go. This allows combination usage of AllowRequest and hystrix.Do/Go.

[swamvenk]

LGTM