This moves endpoint refresh from the context of the service `apid` in
`machined` into `apid` service itself for the workers. `apid` does
initial poll for the endpoints when it boots, but also periodically
polls for new endpoints to make sure it has accurate list of `trustd`
endpoints to talk to, this handles cases when control plane endpoints
change (e.g. rolling replace of control plane nodes with new IPs).
Related to #3069Fixes#3068
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
