PHP-1022: Removed commandCursor in favour of aggregateCursor.

It now should stay on collection, next to aggregate. It does not support a
varargs pipeline.
Merged pull request #631
Added the connection hash that was used while running a command to its output.

We are adding this as a subitem of the $php key, as we can not rely on f.e.
"hash" to be added by the server at some point. Hence we namespace with $php.
It also adds this for *every* command to be consistent. Of course, right now,
it's going to be useful only for MongoCommandCursor::fromDocument, which is
coming in the follow up PR.
Merged pull request #635
Refactor cursor info gathering functions into one.
Add MongoCommandCursor::createFromDocument

This is the implementation for what we discussed during our meeting.
Merged pull request #636
PHP-972: Implement MongoCollection::parallelCollectionScan().

Mostly based on Hannes' code in
https://github.com/mongodb/mongo-php-driver/pull/629
Merged pull request #637