Add functionality that allows eagerCursor(true) on query builders

[jwage]

When used it will return a different type of cursor that eagerly fetches all documents when you start iteration instead of one document per iteration. So it loads all data in to memory before starting iteration.

Without eager cursor:

$qb = $this->conn->selectCollection('dbname', 'collection')->createQueryBuilder();
$cursor = $qb->getQuery()->execute();
foreach ($cursor as $document) { // only loads one $document at a time
}

With eager cursor:

$qb = $this->conn->selectCollection('dbname', 'collection')->createQueryBuilder();
$qb->eagerCursor(true);
$cursor = $qb->getQuery()->execute();
foreach ($cursor as $document) { // loads all documents in to memory before iterating
}

This will tie in to #40 where we have retry functionality. In order to benefit from retries on cursors, you will have to use an eager cursor so that the data fetching is all done before iteration starts so that if a cursor error happens you can try again with a new cursor.

[jmikola]

The benefit of this vs. using toArray() is that object hydration can still be done on-demand, but the cursor's raw data from MongoDB is exhausted in the first iteration, correct?

[jwage]

Actually it doesn't behave that way right now. It fetches the data and hydrates together.

[jwage]

I might tweak it to work that way so hydration is still on demand but the data is pulled all at once.

[jwage]

Actually thinking about it I'm not going to do that as it doesn't really give much of an added benefit. If we conclude otherwise later I can enhance it.