Improve 20180523042841_rename_merge_requests_allow_maintainer_to_push.rb for GitLab.com scale

10.8 RC6 introduces 20180523042841_rename_merge_requests_allow_maintainer_to_push.rb, which just calls:

rename_column_concurrently :merge_requests, :allow_maintainer_to_push, :allow_collaboration

We have close to 10 million merge request rows in production.

This is going to cause enormous replication delay on GitLab.com. We can either:

1. Keep it as-is, and count on the load balancing code to handle it
2. Change this migration to a background migration
3. Revert it entirely (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19061)
4. Get rid of this migration entirely and leave the column names alone

/cc: @yorickpeterse, @abrandl, @smcgivern, @filipa, @mayra-cabrera, @marin, @lulalala