There are cases where the extension registration becomes invalid
(empty rdb files) which leads to extensions not starting up,
although installed and active.
Syncing extension repos on startup fixes that.

The performance impact is negligible (1ms in my measurements).

Change-Id: I10475ce6a054b43c4591b17a8486648ca6a30068
Reviewed-on: https://gerrit.libreoffice.org/61943
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>