Add couch schema version to some database on platform, avoid starting up if that is not there

We want to prevent soledad from running new code on unmigrated databases, and one way to do this is:

• migration script creates something that contains the couch schema version on the platform database
• soledad checks for that information and refuses to startup if the schema version is incorrect

(from redmine: created on 2016-08-29)

• Relations:
  • relates #8420 (closed)