Make the default max RT value TIME_DROP_VALVE configurable

[cdfive]

Describe what this PR does / why we need it

根据应用实际RT情况以及监控数据准确性需求，让MAX RT(即TIME_DROP_VALUE)可配置。

Does this pull request fix one issue?

Fixes #276

Describe how you did it

在SentinelConfig中增加了1个参数配置项csp.sentinel.statistic.max.rt，增加默认值和读取方法

Describe how to verify it

add unit test

Special notes for reviews

1.把CHARSET、COLD_FACTOR的默认值也修改为用常量定义（感觉这样看着一致些）;
2.读取某个特定参数值方法catch里RecordLog.info改为了warn(因为在catch里打印)；
3.cold_factor之前在ColdFactorProperty类读取的，因为默认值在SentinelConfig定义，
改为了用SentinelConfig.coldFactor()读取，其中：
if (coldFactor <= 1) {
coldFactor = 3;
RecordLog.info("cold factor should be larger than 3");
}
判断的是<=1，打印的却是大于3，信息不一致；
这段放到了SentinelConfig.coldFactor()方法里，
判断<=1没变，把信息内容改了下；
4.Constants中的TIME_DROP_VALVE，final修饰去掉了
跟下面那个ON开关一样，便于运行时可以修改，比如以后考虑通过CommandHandler方式。

[codecov-io]

Codecov Report

Merging #292 into master will increase coverage by 7.59%.
The diff coverage is 68.42%.

@@             Coverage Diff              @@
##             master     #292      +/-   ##
============================================
+ Coverage     38.25%   45.85%   +7.59%     
+ Complexity      937      921      -16     
============================================
  Files           218      181      -37     
  Lines          6966     5749    -1217     
  Branches        948      829     -119     
============================================
- Hits           2665     2636      -29     
+ Misses         3964     2780    -1184     
+ Partials        337      333       -4

Impacted Files	Coverage Δ	Complexity Δ
.../sentinel/slots/block/flow/ColdFactorProperty.java	0% <0%> (ø)	0 <0> (ø)	⬇️
.../main/java/com/alibaba/csp/sentinel/Constants.java	85.71% <100%> (+2.38%)	1 <0> (ø)	⬇️
...om/alibaba/csp/sentinel/config/SentinelConfig.java	56.71% <70.58%> (+5.89%)	13 <4> (+3)	⬆️
...aba/csp/sentinel/adapter/servlet/CommonFilter.java	66.66% <0%> (-7.76%)	6% <0%> (-3%)
...a/csp/sentinel/slots/statistic/base/LeapArray.java	61.42% <0%> (-6.93%)	18% <0%> (-6%)
...java/com/alibaba/csp/sentinel/util/AssertUtil.java	8.33% <0%> (-5%)	1% <0%> (-1%)
...ntinel/slots/block/flow/param/ParameterMetric.java	23.33% <0%> (-4.61%)	7% <0%> (-1%)
...alibaba/csp/sentinel/node/metric/MetricWriter.java	56.79% <0%> (-1.79%)	19% <0%> (-2%)
.../slots/statistic/metric/HotParameterLeapArray.java	81.39% <0%> (-1.54%)	11% <0%> (ø)
...k/flow/controller/WarmUpRateLimiterController.java	68.75% <0%> (-0.95%)	4% <0%> (-1%)
... and 105 more

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[sczyh30]

LGTM

[sczyh30]

Cool! Thanks for contributing!