the caching layer on endpoint is redundant.

Here are the 3 related objects in picture:
devicemanager <-> endpoint <-> plugin

Plugin is the source of truth for devices
and device health status.

devicemanager maintain healthyDevices,
unhealthyDevices, allocatedDevices based on updates
from plugin.

So there is no point for endpoint caching devices,
this patch is removing this caching layer on endpoint,

Also removing the Manager.Devices() since i didn't
find any caller of this other than test, i am adding a
notification channel to facilitate testing,

If we need to get all devices from manager in future,
it just need to return healthyDevices + unhealthyDevices,
we don't have to call endpoint after all.

This patch makes code more readable, data model been simplified.