API to explicitly teardown a client
Currently, geoclue relies on the associate D-Bus peer to vanish to know when to destroy the client. This works fine for most applications that use the client for their entire lifetime but with apps like gnome-settings-daemon, this ends up us keeping its client around forever and hence geoclue process keeps running forever. Instead of expecting apps to use a unique D-Bus connection for geoclue, let's provide them an API to explicitly destroy their client when they no longer need to use geoclue.