CAN's store should listen to SCRATCHFILE_ADDRESSED

When CAN's state store detects a SCRATCHFILE_ADDRESSED event, it should update the current_scratchfile_address field. The CAN store's current_scratchfile_address field holds the address of everything above-the-fold in the scratchfile

Backlinks

• When a CAN scratchfile is addressed, check to see if the note is new
  • CANs note-existence handler should listen for SCRATCHFILE_ADDRESSED and emit NOTE_EXISTENCE. CAN's store should listen to SCRATCHFILE_ADDRESSED and NOTE_EXISTENCE