Fix deadlock on unloading LSP module

[rchl]

On unloading LSP module (when saving LSP.boot, for example), ST re-creates all ViewEventListeners referenced from that module. That should result in old instances being garbage collected but in our case DocumentSyncListener was not deleted because settings.add_on_change listener added in DocumentSyncListener.__init__ was not unregistered so it held a strong reference to DocumentSyncListener. That resulted in a deadlock in some conditions (related bug report: sublimehq/sublime_text#6868).

Go through all view listeners on LSP's plugin_unload and call our DocumentSyncListener.before_destroy() so that the settings listener is unregistered which then allows the instance to be GCed.

Also go through ClientConfigs' newly added remove_all_configs() to unregister settings listeners for removed configs instead of just clearing internal lists.