Upsert-type functionality for existing ORM Entities

This is an attempt to fix the issue: https://github.com/FriendsOfSymfony/FOSElasticaBundle/issues/526

It will cause a significant slowdown in a large batch, but it appears to be the only way to prevent an exception from bubbling up during a normal use case.
Merge pull request #527 from baggachipz/master

Upsert-type functionality for existing ORM Entities