Roster issues when contact registers a new key

[daniele-athome]

When a contact registers a new key, the server invalidates the roster from the previous account (i.e. sends unsubscribe to all buddies). This causes some weird behaviour in sending/receiving messages because, if the contact hasn't connected yet, the unsubscribe was not delivered yet, so it didn't happen.
This also mean that for a moment the two users are authorized, which can't happen. This part needs also a fix on the server.

There is also another issue with key fingerprint mismatch that is causing the users database to have 2 records with different fingerprint and same timestamp (which can't happen).

Issue #1127 might be somewhat related.