keep <nick/> for presence subscription stanzas

when the contact is offline, the presence subscription stanza will be stored and resend when the contact is online. but it doesn't store the original packet; only part of the stanza will be stored and others will be lost.
this commit doesn't address this issue entirely; it just stores one more element - the `<nick/>` element
and it passes the original packet to the handler of the `roster_in_subscription` hook which makes it possible for the handler to store the original packet in the future
one more thing, we will only supports odbc, so `mod_roster` and `mod_shared_roster_ldap` are not maintained