CI Improvements

This PR adds some improvement to our continuous integration environment setup.

* It separates builds for opcaches and databases allowing more control and isolation.
* Runs just the database related test for database builds.
* Updates the version of XCache and runs tests with it (no builds were configured with XCache before)
* Reuses PHP extensions already provided by the CI env where possible.
* Disables any notifications. (GitHub provides still enough information on i.e. PRs for that, also stops spamming channel with user builds)
* Adds the ability to enable APCu in the future.
* Fixes two expectations in mongo tests that weren't revealed in past test runs.
* Consolidates database configurations into one file.
* Blacklists APC for PHP 5.5.