extend archiver to pull L1 -> L2 events from message box

[LHerskind]

The Archiver must:

1. Fetch messages that have been inserted into the Inbox
2. Store those in memory in an "Pending L1 to L2 messages" (store the full event data with fee etc)
   • For now memory is fine, over time will have to be stored in database, possibly consult how l2blocks are stored to get initial idea.
3. When L2Blocks are fetched, remove entries from the pending set as they are now spent. Beware that duplicates are allowed and should be accounted for by the archiver.