Fixed a fatal "Allowed memory size exhausted" error when updating the plugin. Updater::clearCache() runs on delete_site_transient_update_plugins and then deletes an update transient of its own, which fires the generic deleted_site_transient and deleted_option hooks; any listener that refreshes the plugin update cache in response re-entered the deletion hook, and the two recursed until PHP ran out of VM stack. clearCache(), injectUpdate(), and the remote fetch in metadata() now each hold a re-entry guard.
Stopped Updater::metadata() from repeating the license-server request when the update transient filter re-enters before the first response is cached.
Fixed the Redis object-cache drop-in reporting a successful delete for a key it never held. Core's WP_Object_Cache::delete() returns false in that case, and delete_site_transient() fires the generic deleted_site_transient hook only on a true result, so the unconditional answer re-dispatched that hook on every repeat deletion. This is what kept the update-cache recursion above from settling on sites where the drop-in is installed without a reachable Redis server.
