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.

examples/repl/cmdline.rs

@@ -13,6 +13,7 @@ use deltachat::dc_receive_imf::*;
 use deltachat::dc_tools::*;
 use deltachat::imex::*;
 use deltachat::location;
+use deltachat::log::LogExt;
 use deltachat::lot::LotState;
 use deltachat::message::{self, ContactRequestDecision, Message, MessageState, MsgId};
 use deltachat::peerstate::*;
@@ -508,7 +509,7 @@ pub async fn cmdline(context: Context, line: &str, chat_id: &mut ChatId) -> Resu
             context.maybe_network().await;
         }
         "housekeeping" => {
-            sql::housekeeping(&context).await;
+            sql::housekeeping(&context).await.log(&context);
         }
         "listchats" | "listarchived" | "chats" => {
             let listflags = if arg0 == "listarchived" { 0x01 } else { 0 };