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.

This error raises while getting info on target (or server) replica. On
previous implementation there was nothing to do here, but now that we
have db creation in place this error should be handled just like u1db
original implementation.
The reason is that db creation occurs during sync exchange, but before
this we try to ask for info and the code that checks for info raises an
error that will be used to signal the client that a database will be
created and that it must save the replica uid returned by server, after
sync exchange (where we send/fetch documents).

this is a workaroud to reduce the chances of failed sync due to
timeouts. this should be properly tackled by:

1. implementing proper cancellable for the sync operation.
2. implementing a retry count at the level of a single request, handled
internally by soledad.

in this way we can remove the retries logic from the soledadbootstrapper
in the bitmask client.

- Related: #7382

- 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.

This tests the previous fix on ensuring a db that is missing a doc other
than 'docs'.

This code only checks for 'docs' presence, while we have 3 design
documents. If one of them is missing, but 'docs' is not, then it will
not ensure the others.
This is needed to properly ensure ddocs on create command line script.