config_cache fixes (#3145)

* add simple backup export/import test

this test fails on current master
until the context is recrated.

* avoid config_cache races

adds needed SQL-statements to config_cache locking.

otherwise, another thread may alter the database
eg. between SELECT and the config_cache update -
resulting in the wrong value being written to config_cache.

* also update config_cache on initializing tables

VERSION_CFG is also set later, however,
not doing it here will result in bugs when we change DBVERSION at some point.

as this alters only VERSION_CFG and that is executed sequentially anyway,
race conditions between SQL and config_cache
seems not to be an issue in this case.

* clear config_cache after backup import

import replaces the whole database,
so config_cache needs to be invalidated as well.

we do that before import,
so in case a backup is imported only partly,
the cache does not add additional problems.

* update CHANGELOG

src/sql/migrations.rs

@@ -36,6 +36,13 @@ pub async fn run(context: &Context, sql: &Sql) -> Result<(bool, bool, bool, bool
             Ok(())
         })
         .await?;
+
+        let mut lock = context.sql.config_cache.write().await;
+        lock.insert(
+            VERSION_CFG.to_string(),
+            Some(format!("{}", dbversion_before_update)),
+        );
+        drop(lock);
     } else {
         exists_before_update = true;
         dbversion_before_update = sql