perf: optimize copy on write attributes

[yuzawa-san]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

none

Describe the solution you've provided

i have modified the copy on write mechanism for custom attributes to avoid having to copy the whole attribute map. instead i chain to the parent map. this does not change the public API or outwards behavior. the implementation was complicated because of the need to support the "remove" operation. additionally, i leveraged the equals and hashCode of Map. note: hashCode from Map is not order dependent since it uses addition across all of the entries and addition is commutative.

Describe alternatives you've considered

i was also playing around with a lazy LDValue provider such that we do not need to create all of the attributes if we do not use them but that seemed like a lot of rework and hard to do in a codebase currently targeting jdk 7.

Additional context

i was doing flame graph analysis and found that there were a lot of copies of the hash maps going on, so i thought i could try to eliminate those copies. my use case involves the case in the new test case i added, but it is located in a hotspot in my codebase. the parent context has dozens of attributes and is shared across multiple evaluations. however the child contexts add on only a handful of attributes. the current implementation was copying over all of the attributes for each new build since the parent context was tainted. this root/parent chaining and reuse is a common paradigm over the codebase

controller(){
 LDContext rootContext = LDContext.builder(key).set(...).set(...)......build();
 ...
 if (ldClient.boolVariation(rootContext, ...)) { ... }
 ...
 for(a lot of iterations) {
  LDContext innerContext = LDContext.builderFromContext(rootContext).set(....).build()
  if(ldClient.boolVariation(innerContext, ...)) { ... }
  // LDContext.builderFromContext(innerContext) may be called as well
  ...
 }
 ...
}