no more stalling while waiting for a disco info while sending a message.

We need to check if the remote entity supports 0184, but if it doesn’t
support disco#info, then we will get an iq type="error" and nothing
will be cached, leading to disco#info queries being sent each time.

Keep a cache valid 2 hours of the JIDs which replied with an error.
TODO: check that this the kind of time period we want.

xmpp.plugin['xep_0030'].has_identity appears to be unreliable at best,
so we provide our own.

Might help the case of carbons not displayed.