Since failCommand is only available on newer server versions and need a special parameter to be enabled, the configureFailPoint helper now automatically skips a test if failCommand is not available, instead of failing the test with an error.

For most tests, we only want to expose a single mongos node to avoid issues with fail points. This change parses and reassembles the connection URI to only return a single mongos node in the URI.

This fixes issues in tools like PhpStorm that are not able to show test progress when the test name changes.

Merged from $647

https://jira.mongodb.org/browse/PHPLIB-362

Note that I haven't added any `use` statements for new classes. This will be done automatically when we add `phpcs` checks in [PHPLIB-330](https://jira.mongodb.org/browse/PHPLIB-330).

Merged from #646

https://jira.mongodb.org/browse/PHPLIB-401

Merged from #645.

* Don't collect unexpected extra commands in spec tests: as discussed in https://github.com/mongodb/mongo-php-library/pull/642#discussion_r307108104, it makes more sense to not collect commands that we're not going to be asserting against instead of dropping them later.
* While the docs for the aggregation helpers in `MongoDB\Database` and `MongoDB\Collection` have been updated with respect to read concerns and write stages, the documentation in the `MongoDB\Operation\Aggregate` wasn't. This is done in this PR.