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

CHANGELOG.md

@@ -19,7 +19,7 @@
 - place common headers like `From:` before the large `Autocrypt:` header #3079
 - keep track of securejoin joiner status in database to survive restarts #2920
 - remove never used `SentboxMove` option #3111
-- improve speed by caching config values
+- improve speed by caching config values #3131 #3145
 - optimize `markseen_msgs` #3141
 - automatically accept chats with outgoing messages #3143