When a nomad client restarts/upgraded, nomad restores state from running
task and starts the sync loop.  If sync loop runs early, it may
deregister services from Consul prematurely even when Consul has the
running service as healthy.

This is not ideal, as re-registering the service means potentially
waiting a whole service health check interval before declaring the
service healthy.

We attempt to mitigate this by introducing an initialization probation
period.  During this time, we only deregister services and checks that
were explicitly deregistered, and leave unrecognized ones alone.  This
serves as a grace period for restoring to complete, or for operators to
restore should they recognize they restored with the wrong nomad data
directory.