CAN's store should listen to NOTE_EXISTENCE

When CAN's state store detects a NOTE_EXISTENCE event it should update the current_scratchfile_new field if the event address matches address of current_scratchfile_address. The CAN store's current_scratchfile_new field holds if the current scratchfile exists in the notebook.

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