`FailureRatio` and `SamplingDuration` parameters of Polly V8 circuit-breaker

[RaynaldM]

Add two parameters in QoSOptions:

• FailureRatio: The failure-success ratio that will cause the circuit to break/open.
• SamplingDuration: The time period over which the failure-success ratio is calculated (in seconds).

The aim is to take better advantage of Polly's circuit-breaker possibilities, and to manage circuit-break conditions more finely, on a case-by-case basis.

Currently, these 2 parameters are set by default (and in hard coded) with an error ratio of 80% and a sample duration of 10 seconds. These values may not suit everyone.