Zero time upgrade for Bitbucket DC

Now Bitbucket DC does not allow to upgrade to recent versions (major release or minor release) without downtime. We need 0-time upgrade for our production environment.

Possible way to solve it:

Let's image that we have BB DC running on version 4.8.0 on 2 nodes:

• Upgrade one node to 4.8.1, now BB DC have only one working node on 4.8.0, server still work as 4.8.0. Node started on 4.8.1 take pause until database complete migration.
• Then database starts migration, Bitbucket pass into readonly mode, disallowed database write, also disallow Git operations except git clone/git fetch to clients.
• After database migrated, node on 4.8.0 became to invactive and mometally node on 4.8.1 starts working, Bitbucket now switched to new version and starts working in full mode.
• Other nodes upgrades in usual mode.