`init_script_path` option can be used in `[DEFAULTS]` to run a script at the end of
backend initialisation. A new `init_pre_script` method is used to wait for backend to
reach this stage (designed to be used mostly by CLI frontend), then the usual `ready_get`
method is finished once the script is finished.
folloing packages refactoring, legacy pickled values could not be unpickled (due to use of
old classes). This temporary workaround fix it, but the right thing to do will be to move
from pickle to JSON at some point.