MissingDesignDocError raised on get_sync_info due to a missing design
document will be handled by the server during sync.
Ensure is now False by default, and thus database creation can deliver
an empty one that will be ensured during sync, following the ensure
parameter.

- Related: #7503

- bug: we were dumping the received secrets locally to disk *before*
  setting the received property for the active secret, and therefore the
  'active_secret' was always marked as null.
- refactor common code into an utility method.

- Resolves: #7412

From:
[refactor] removing getters and setters from couch.py
_couch_url was a private variable with getter and setter doing the same
as a public variable. It is accessed all over the code, so being private
with getters and setters didnt make sense. This commit fixes the tests
to also follow this style from now on.