Handoff sender sends sync periodically

Equivalent to #560, but targetted at the 2.0 branch.
The following description was copied over from #560:

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.