Make the value of RT_MAX_EXCEED_N in DegradeRule configurable

[cdfive]

Issue Description

In the process of degrade, when resource enters a quasi-degraded state, the max count (the value of RT_MAX_EXCEED_N in DegradeRule class) which statistics the next coming requests passed now is fixed value: 5.

Users in community are wondering that why the value is fixed.

Describe what happened (or what feature you want)

Users may expect that the RT_MAX_EXCEED_N can be configured in different scenes.

How about adding a property like csp.sentinel.degrade.rt.max.exceed.count?
and does it need to be modified dynamically?

Anything else we need to know?

Related to #154

[sczyh30]

I think this can be different per rules. We may need to add a new field for this, but this should be carefully designed, as this RT_MAX_EXCEED_N has multiple usages actually. In exception ratio mode, if total QPS < RT_MAX_EXCEED_N, even if the exception ratio exceeded the threshold, the request would pass.

[sczyh30]

Resolved in #789. It's now configurable as two attributes:

• rtSlowRequestAmount for the minimum number of consecutive slow requests that can trigger RT circuit breaking.
• minRequestAmount for the minimum number of requests (in an active statistic time span) that can trigger circuit breaking.