clonebundles: advertise clone bundles feature to clients Server operators that have enabled clone bundles probably want clients to use it. This patch introduces a feature that will insert a bundle2 "output" part that advertises the existence of the clone bundles feature to clients that aren't using it. The server uses the "cbattempted" argument to "getbundle" to determine whether a client supports clone bundles and to avoid sending the message to clients that failed the clone bundle for whatever reason.