Monitor Bot: Convert non-str keys to str when using join()

string.join() only accept strings...
Removed an annoying trace
Peer access update is notified only if its value actually changed

Also added some thread safety for set_access and unset_access
Directory.register() supports Peer access update

If Directory.register() is called for an already known peer, the given peer accesses are added/updated in the Peer bean, instead of being rejected.
This allows to have two transports with two different discovery channels (namely both HTTP and XMPP transports active), and to merge their registration information.