Properly handle default values for allowMaximumSizeToDivergeFromCoreSize

[ptab]

Properly handle default values for allowMaximumSizeToDivergeFromCoreSize when configuring properties using Archaius.

Fixes #1434

[ptab]

To give a bit more context: the default value for HystrixThreadPoolProperties.allowMaximumSizeToDivergeFromCoreSize is false instead of null, which means

private static boolean getValueOnce(String propertyPrefix, HystrixThreadPoolKey key, String instanceProperty, boolean builderOverrideValue, boolean defaultValue) {
  return forBoolean()
         .add(propertyPrefix + ".threadpool." + key.name() + "." + instanceProperty, builderOverrideValue)
         .add(propertyPrefix + ".threadpool.default." + instanceProperty, defaultValue)
         .build()
         .get();
}

will never try to read the value for threadpool.default.

A workaround for the current code is to either configure it programatically using HystrixThreadPoolProperties.Setter or adding a configuration for every specific threadpool key.

[mattrjacobs]

I just built a jar with this PR and deployed it into an environment with hystrix-1.5.8. It has a regression - when the property is set to false, the default maximum (10) is getting picked up. I'm reviewing the code to determine where that happens.

[mattrjacobs]

@ptab Can you try out #1447 and see if it works for you? It builds on top of this PR (#1435)

It fixes 2 issues:

1. Maintain invariant of maximumSize == coreSize if allowMaximumSizeToDivergeFromCoreSize == false
2. Fix Configuration stream so that it also knows about this invariant

[ptab]

I am a bit late to the party, but I did run 1.5.9 against my test case and verified that it works as expected. Thanks for accepting it! 👍

[mattrjacobs]

Thanks for the contribution!