Copy constructors should copy mutable MergePolicyConfig objects and not share them

[pivovarit]

Some of the existing copy-constructors are introducing shared mutability by reusing existing config objects instead of creating fresh copies which can lead, for example, to unsolicited changes in default configurations.

Config config = new Config();
ScheduledExecutorConfig newConfig = config.getScheduledExecutorConfig("nonexisting");
ScheduledExecutorConfig defaultConfig = config.getScheduledExecutorConfig("default");

newConfig.getMergePolicyConfig().setPolicy("spanish inquisition!");

defaultConfig.getMergePolicyConfig().getPolicy(); // spanish inquisition!

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Related: #17874