Fixes a bug where an allocation is considered healthy if some of the
tasks are being restarted and as such, their checks aren't tracked by
consul agent client.
Here, we fix the immediate case by ensuring that an alloc is healthy
only if tasks are running and the registered checks at the time are
healthy.
Previously, health tracker tracked task "health" independently from
checks and leads to problems when a task restarts. Consider the
following series of events:
1. all tasks start running -> `tracker.tasksHealthy` is true
2. one task has unhealthy checks and get restarted
3. remaining checks are healthy -> `tracker.checksHealthy` is true
4. propagate health status now that `tracker.tasksHealthy` and
`tracker.checksHealthy`.
This change ensures that we accurately use the latest status of tasks
and checks regardless of their status changes.
Also, ensures that we only consider check health after tasks are
considered healthy, otherwise we risk trusting incomplete checks.
This approach accomodates task dependencies well. Service jobs can have
prestart short-lived tasks that will terminate before main process runs.
These dead tasks that complete successfully will not negate health
status.
