rework __heartbeat__ log message - heartbeat failure is a .warn(), not an error().  the latter indicates an unexpected condition that should never happen in production.  thought here is that the dependee that's failing should have errors() in her log message, and the depener should have warnings.  Also, this gets error messages out of the console output of the unit test.

for __heartbeat__ use an application level global timeout to catch the situation where the other end is not reachable and ensure total wait time is <= 5s (in this case, no 'connect' event is ever emitted)

Handle connection errors and timeouts correctly on heartbeat dependency checks and send error in response - issue #1767