feat: Make ParseObject availableKeys Set a thread safe synchronizedSet

[canuludag]

ParseObject gives a ConcurrentModificationException when concurrently modifying the availableKeys HashSet. By making availableKeys synchronized set we can avoid these kind of exceptions.
Looks like there are some open issues that can be related to this -> #154

[codecov]

Codecov Report

Merging #795 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #795   +/-   ##
=========================================
  Coverage     53.35%   53.35%           
  Complexity     1749     1749           
=========================================
  Files           132      132           
  Lines         10268    10268           
  Branches       1426     1426           
=========================================
  Hits           5478     5478           
  Misses         4337     4337           
  Partials        453      453

Impacted Files	Coverage Δ	Complexity Δ
Parse/src/main/java/com/parse/ParseObject.java	50.95% <100%> (ø)	205 <0> (ø)	⬇️

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[vovkab]

Parse SDK: 1.17.3

I think as a result of this change we are now seeing a new crash:

Fatal Exception: java.lang.StackOverflowError
    at java.util.HashMap.secondaryHash(HashMap.java:350)
    at java.util.HashMap.put(HashMap.java:404)
    at java.util.HashSet.add(HashSet.java:95)
    at java.util.Collections$SynchronizedCollection.add(Collections.java:380)
    at java.util.Collections$SynchronizedCollection.add(Collections.java:380)
    ...
    at java.util.Collections$SynchronizedCollection.add(Collections.java:380)
    at java.util.Collections$SynchronizedCollection.add(Collections.java:380)
    at java.util.Collections$SynchronizedCollection.add(Collections.java:380)
    at com.parse.ParseObject$State$Init.<init>(ParseObject.java:133)
    at com.parse.ParseObject$State$Builder.<init>(ParseObject.java:246)
    at com.parse.ParseObject$State.newBuilder(ParseObject.java:301)
    at com.parse.ParseObject.mergeFromServer(ParseObject.java:982)
    at com.parse.ParseObject.fromJSON(ParseObject.java:707)
    at com.parse.ParseObject.fromJSON(ParseObject.java:684)
    at com.parse.ParseLiveQueryClientImpl.handleObjectEvent(ParseLiveQueryClientImpl.java:322)
    at com.parse.ParseLiveQueryClientImpl.parseMessage(ParseLiveQueryClientImpl.java:253)
    at com.parse.ParseLiveQueryClientImpl.access$000(ParseLiveQueryClientImpl.java:24)
    at com.parse.ParseLiveQueryClientImpl$1.call(ParseLiveQueryClientImpl.java:201)
    at com.parse.ParseLiveQueryClientImpl$1.call(ParseLiveQueryClientImpl.java:199)
    at bolts.Task$4.run(Task.java:357)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
    at java.lang.Thread.run(Thread.java:841)

Possible issues:

1. We probably should not be synchronizing everywhere, because as a result, we would get synchronizedSet(synchronizedSet(synchronizedSet(...))
2. If we use synchronizedSet() we should have copied original set instead of wrapping it as is.
3. We do not synchronize set created in a second constructor:

com.parse.ParseObject.State#State(android.os.Parcel, java.lang.String, com.parse.ParseParcelDecoder)

/* package */ State(Parcel parcel, String clazz, ParseParcelDecoder decoder) {
    ...
    availableKeys = new HashSet<>(available);
}

Are we relying on a side effect that someone accidentally would synchronize it for us?

[RodrigoSMarques]

Hello. I have the same however only on devices with Android 4.x. In no other version do I have problems.