Fetch/sync API repeats cursor with more-to-follow

[jessepeterson]

Reported in micromdm/micromdm#1016 that Apple repeatedly sends a more-to-follow response even for the same sync token that is returned. We'd need to address that in NanoDEP as well as we also quickly cycle out of our loop for the more-to-follow attribute:

nanodep/sync/syncer.go

Lines 216 to 217 in 0ff451e

	if resp.MoreToFollow {
	continue