Add separate mongo driver for PHP 7 using the userland library

[sagikazarmark]

Q	A
Bug?	no
New Feature?	yes
Links	Moved from bernardphp/bernard#316

[GromNaN]

@acrobat Do you still need to update the mongodb driver for bernard?

[jmikola]

Some observations:

• bernardphp/bernard@7648172 added mongodb/mongodb to suggested packages, although the integration at the time was based on the now-unmaintained ext-mongo extension (see: src/Driver/MongoDBDriver.php
• Somehow, mongodb/mongodb remained even when other suggestions were removed in bernardphp/bernard@21ebde7
• Since PHP 7 support was dropped in bernardphp/bernard@dedbcf6, there's no longer any concern about needing to support multiple drivers (as mentioned in Compatibility with mongodb php7 extension bernard#316 (comment)).
• Compatibility with mongodb php7 extension bernard#316 contrasted ext-mongo and ext-mongodb. While ext-mongodb does only provide a low-level API, the mongodb/mongodb userland library is on par with what existed in ext-mongo. Porting the current driver (src/Driver/MongoDB/Driver.php) to use the new library wouldn't take too much effort (and the upgrade guide) could help).