Also drop support for Mongo 2.4 (PHPLIB-282)

* PHPLIB-289 ChangeStreams Examples for Docs

* Add ChangeStream documentation

* PHPLIB-276 Change stream support

* PHPLIB-297 Support maxTimeMS option for DropIndexes operation

* PHPLIB-254 Support maxTimeMS option for CreateIndexes operation

* PHPLIB-293 Add support for a comment parameter to the aggregate command

The call is made following the MongoDB\Client::selectCollection() signature instead of the MongoDB\Database::selectCollection() signature

The call is made following the MongoDB\Client::selectCollection() signature instead of the MongoDB\Database::selectCollection() signature

The call is made following the MongoDB\Client::selectCollection() signature instead of the MongoDB\Database::selectCollection() signature

This will allow the returned CollectionInfoIterator and IndexInfoIterator classes to be rewound and iterated multiple times.

Ensure that a call to CachingIterator::rewind() no longer exhausts results up front, which would impose a BufferedIterator design.

Initializing the cache to an empty array also ensures that we can count an empty result set.