This introduces the ability for status notes to handle community
history archive magnetlinks. To make this work, a few things are needed:
1. A new database table has been introduced to store message archive
hashes. This is necessary so status nodes can determine whether or
not they need to download a certain archive
2. The messenger's `handleRetrievedMessages()` has been exteded to take
magnetlink messages into account
3. New APIs were added to download torrent data given a magnetlink and
also to extract messages from downloaded archives, which are then
later fed to `handleRetrievedMessages`
Closes#2568
