GH-3900: misc mem improvements

[arne-bdt]

GitHub issue resolved #3900

Pull request Description:

Minor bugs fixed:

• FastArrayBunch does not hold references to orphans after #tryRemove and #removeUnchecked
• FastHashMap#computeIfAbsent does not creat an invalid state if call of absentValueSupplier fails
• FastHashMap#compute now grows on insert like all other insert operations of this map

Improvements:

• iterators and spliterators don't need a runnable for concurrency check any more. They check the size directly, which has been shown to be faster.
• FastHashBase#fillPositionsArray now iterates over the dense keys array, which should be faster in most cases.
• Removed FastHashSet#IndexedKey and the corresponding iterator and spliterator implementations --> introduced FastHashBase#forEachKey and #forEachKeyParallel instead
• added JavaDoc
• added tests

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• Tests are included.
• Commits have been squashed to remove intermediate development commit messages.
• Key commit messages start with the issue number (GH-xxxx)

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