Fix imex race condition, (#2255)

fix #2254: if the DB was closed without calling stop_io() and then an interrupt arrives (e.g. incoming message), the db was corrupted.

* Add result.log() for logging with less boilerplate code

* Bugfix: Resultify housekeeping() to make it abort if the db is closed instead of just deleting everything

* Require the UI to call dc_stop_io() before backup export

* Prepare a bit better for closed-db: Resultify get_uidvalidity and get_uid_next and let job::load_next() wait until the db is open

About the bug (before this PR):
if the DB was closed without calling stop_io() and then an interrupt arrives (e.g. incoming message):
- I don't know if it downloads the message, but of course at some point the process of receiving the message will fail
- In my test, DC is just in the process of moving a message when the imex starts, but then can't delete the job or update the msg server_uid
- Then, when job::load_next() is called, no job can be loaded. That's why it calls `load_housekeeping_job()`. As `load_housekeeping_job()` can't load the time of the last housekeeping, it assumes we never ran housekeeping and returns a new Housekeeping job, which is immediately executed.
- housekeeping can't find any blobs referenced in the db and therefore deletes almost all blobs.

deltachat-ffi/deltachat.h

@@ -1858,6 +1858,7 @@ dc_contact_t*   dc_get_contact               (dc_context_t* context, uint32_t co
 
 /**
  * Import/export things.
+ * During backup import/export IO must not be started, if needed stop IO using dc_stop_io() first.
  * What to do is defined by the _what_ parameter which may be one of the following:
  *
  * - **DC_IMEX_EXPORT_BACKUP** (11) - Export a backup to the directory given as `param1`.