PHPLIB-609: Allow CachingIterator to handle Iterators with non-unique keys

[aleksandr-rudo]

https://jira.mongodb.org/browse/PHPLIB-609

[jmikola]

Additionally, please add a new test case to CachingIteratorTest that shows the class can now support iterators that return duplicate keys. The existing helper methods in that class (e.g. getTraversable) might not be usable for that purpose, but you can likely get by with creating a generator. For example:

$duplicateKeyGenerator = function() {
    yield 0 => 'a';
    yield 0 => 'b';
    yield 0 => 'c';
};

[jmikola]

One suggestion to rename the count test and add testIterationNonUniqueKeys. I think we'll be good after that.

[jmikola]

LGTM, but please hold off on merging until I branch v1.12.

The one build failure is unrelated to this change and is due to upstream changes in MongoDB 6.0. I added a comment in PHPLIB-810 to track it.