Use stanza async reply instead of listener

[daniele-athome]

In some cases (e.g. last activity request, but all IQs in general) we need to process the reply, not wait with a listener. This is especially useful for handling error packets which are not captured by extension listeners.
This should also solve the big wait during contacts sync when requesting keys to non-Kontalk or to blocked users (because the error reply is not processed in any way).

This can (and should) be considered a bug because it causes a few reliability issues.

Strictly speaking, we'll need to convert:

• last activity
• public key request
• roster match (maybe)

Also there are existing calls to conn.sendIqBlaBla. Unify them into the new method.