jsonrpc: auto restart io on setConfig for the following keys `sentbox_watch`, `mvbox_move` and `only_fetch_mvbox`

[Simon-Laux]

desktop had this in it's backend code and we don't expose startio/stopio in the jsonrpc api yet. also deltachat.h docs say it needs to be done. I added the new restart_io_if_running method into core because I want to minimize possible side effects (only restart the io if it was running before.