Index pushed changes during ES instance downtime

Description

Right now if changes are pushed and the ElasticSearch instance is down these modifications will not be indexed. Even worse, if a project is created during the downtime it will not even appear on the search results. It seems that if the downtime is short enough and the project was already indexed, the changes done during the downtime are still indexed after the ES instance runs again.

Proposal

The indexing process should keep track of the projects that need to be indexed if the ES instance is down. Reindexing everything is not always a solution with thousands of projects as it will take quite long and affect the performance of the Gitlab instance.