* 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
8487eefe46