Implement support for using RosterStorage
Ignore roster pushes with invalid from.

RFC 6121 mandates in Section 2.1.6 that a receiving client MUST
ignore roster pushes if the from attribute is present and does
not match the bare user JID.
Reply to roster pushes

According to the semantics of IQ stanzas (RFC 6120, 8.2.3), the client
must reply to a roster push with a IQ result or IQ error.