When creating new identity, if existing KHID owned by user, get all mail from date KHID was registered.

[dnotestein]

This will give user opportunity to retrieve old mail when re-creating an identity in a new profile (assuming the old mail is still stored in mail server). This is relatively low priority issue for now, just recording it for future.

[dnotestein]

This was implemented by giving user choice of how much history to download when creating a new identity (default is none for now, eventually default could be creation date for registered KHIDs). Eventually this value could also be used to search for any "contact authorization" keys that need regenerating as well. Implemented in e371aad.

Note that since all identities share the last_sync_time, fetching history will re-fetch the history of any identity, not just the recently created one, so right now this will re-fill deleted messages in other identities. One possible way around this issue would be to temporarily set the decrypter into a mode where it only decrypts messages for the new identity, but this idea has problems (what if some new message came in that we do want for another identity, for example).