Refactor config file

[wonder-sk]

It would be useful to do some changes to the config files to make them more portable:

• geodiff_exe - let's remove this option - it is more of a local installation detail rather than a user configuration - dbsync should simply expect geodiff in PATH
• working_dir - let's remove this option - like above, it is a detail of the installation - best to simply use a working dir automatically in a temp folder (probably no need to have env variable to override it)
• init_from - let's add this option with two possible values: gpkg and db - right now users need to specify those manually when running dbsync.py or dbsync_daemon.py, but it makes more sense to have it a part of config

[wonder-sk]

Fixed in #103