Handoff sender sends sync periodically

If we can discover the `handoff_receive_timeout value`, then use 1/3rd that interval to send a sync/heartbeat thingie, otherwise 60 seconds (for infinity & undefined).

Purpose: to avoid unhelpful timeouts when the handoff sender is filtering (e.g. repair or ring resizing handoff) and encounters zillions of keys to filter and thus doesn't send anything to the receiver for a while.  A workaround is to set the receiver's timeout to be extremely long, but that's a kludge IMHO.

It seems like overkill to have to execute this `case` statement for every item in the fold during handoff. Since `start_visit_item_timer` calls `timer:send_interval`, it seems like we could call `start_visit_item_timer` right before starting the fold and cut this function down to just the `receive` block.