MEDIUM: proxy: implement backend deletion

MEDIUM: proxy: implement backend deletion

This patch finalizes "del backend" handler by implementing the proper
proxy deletion.

After ensuring backend deletion can be performed, several steps are
executed. First, any watcher elements are updated to point on the next
proxy instance. The backend is then removed from ID and name global
trees and is finally detached from proxies_list.

Once the backend instance is removed from proxies_list, the backend
cannot be found by new elements. Thread isolation is lifted and
proxy_drop() is called, which will purge the proxy if its refcount is
null. Thanks to recently introduced PROXIES_DEL_LOCK, proxy_drop() is
thread safe.