Fixes to the heartbeat channel...
Fixes to the heartbeat channel
* The heartbeat channel had some erroneous zeromq logic, and entirely False comments (as described in
#967). This has been fixed.
* KernelManager.is_alive() checks if the hb_channel is running if
the kernel is not owned, rather than always returning True.
* BlockingKM's hb_channel has been relaxed to 1s polling, because replies are not
reliably much faster than that. There are occasional >0.5s outlier responses.
