MongoDB support, refactoring

[anyx]

This is variant extension with support mongodb fixtures .

Differences:

• parameter "use_backup" was removed
• separate fixtures executor for each type of database

[megazoll]

This method returns false or null. And there is no usage of return value of this method. I'm think you should write there just return;

[robocoder]

Thanks. I plan to stabilize the 2.5 branch and port it to master before merging this PR.