Issue #109 Upgrade to latest PHP driver

[rgubby]

This is to support the new mongo driver for PHP.

The PHP drivers implement a different API, so all chainable calls such as ->find()->sort()->limit() have been replaced with another format. Read preferences have changed (how you set them), so have some of the function calls. An upgrade guide can be found here: https://mongodb.github.io/mongo-php-library/upgrade-guide/

There are some requirement changes:

• Mongo 2.6.x and above. This has changed from requiring Mongo 2.x - the reason for this is the way an updateOne or replaceOne function works with the current functionality with the new PHP mongo library. If we want to support older versions of Mongo, we'll need to change some functionality.
• PHP 5.4+. The MongoDB PHP library only works with PHP 5.4 and above - so we're tied to this. Also - we've started using some PHP 5.4 and above functionality (array notation for a start).
• Added compatibility for PHP 7.0 (7.1 has unit test failures so I haven't added that).

A preview modification for Tripod involved retrying connections when stepping down and removing servers from the repset. I've gone through the same testing process with this new branch as I did on that PR. All works as expected.

Todo

• [x] Decide if we're going to support BSONDocuments in MongoGraph or leave it as arrays
• [x] db->getLastError() does not work anymore - find a solution
• [x] Look into how to set read preferences correctly rather than how we were doing it in the old driver
• [x] Transaction log and document locking to work
• [x] Replace all instances of Mongo* - not touched the scripts/utils yet so there are some instances.
• [x] Look into how to manage a successful connection without doing a PING command
• [x] Travis support for new mongo driver - remove the old one
• [x] Can you define a mongo version in Travis?
• [x] Check docblocks and type hinting are correct
• [x] Check for all instances of Mongo*

[kiyanwang]

did you check to see if the scripts in scripts/mongo are affected by these changes? in principle the changes should be transparent but is it worth checking?

[rgubby]

did you check to see if the scripts in scripts/mongo are affected by these changes? in principle the changes should be transparent but is it worth checking?

Yes - i checked - there was an amendment made to just one of the scripts but no others - they use Tripod methods to do anything.