Package gitlab-elasticsearch-indexer for GitLab EE 9.1

We're hoping to ship a new elasticsearch indexer in GitLab 9.1: https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer .

The current indexer is a Ruby script included in the gitlab-ee repository.

The new one is a Go project with (currently) a minimum version requirement of go1.7.

Fixing it for go1.6 is easy enough, but go1.5 would be more difficult. I understand we're aiming to allow go1.7 this release, per https://gitlab.com/gitlab-org/gitlab-omnibus-builder/merge_requests/33 and https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2106 , so I'm not doing the backporting work just yet.

The output of make is a single binary: bin/gitlab-elasticsearch-indexer. You can run make install PREFIX=...

We're aiming to support both indexers for 9.1 so that people can switch back to the old indexer if there are problems. GitLab EE support is in this MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1483 . It depends on gitlab-elasticsearch-indexer being in the $PATH of running sidekiq processes.