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.

http://jira.mongodb.org/browse/PHPLIB-460

With these changes, we're preparing the build file to do more than just running tests. To make feedback more immediate and to save resources, the build now runs a smoke screen testing step before running the full build pipeline. This should catch simple errors and will also do static analysis before running all tests. Once smoke screen testing has completed, the full build pipeline will be executed.

This PR also adds testing against PHP 7.4 with the build currently failing due to a deprecation:
```
Function ReflectionType::__toString() is deprecated
/home/travis/build/alcaeus/mongo-php-library/tests/Operation/ExplainTest.php:17
```

This will be fixed at a later step once I've figured out what's causing this.

With this change, the build first runs a single smoke screen build against the latest versions to save on build resources. Remaining topologies, server versions and PHP versions will be tested in parallel if this first build step was successful.

Merged from #644

https://jira.mongodb.org/browse/PHPLIB-438

Merged from #643

https://jira.mongodb.org/browse/PHPLIB-413

Merged from #642.

https://jira.mongodb.org/browse/PHPLIB-402

I've extracted the `is_in_transaction` helper from `MongoDB\Collection` in a separate to functions since we need it more often. I've also decided to extract the options for the aggregate helper to a common file to avoid unnecessary duplication of option descriptions.