Use a channel to communicate enrollment IDs

[jessepeterson]

We can avoid buffering a potentially very large slice of strings by instead returning a channel and letting the receiver deal with it how they will.

kmfddm/storage/storage.go

Lines 30 to 37 in 60d9089

	// RetrieveEnrollmentIDs retrieves MDM enrollment IDs from storage.
	// In the case of sets and declarations the transitive associations
	// are traversed to try and collect the IDs. When multiple slices
	// are given they should be treated like a logical or (i.e. finding
	// all enrollment IDs for any of the given slices).
	// Warning: the results may be very large for e.g. sets (or, transitively,
	// declarations) that are assigned to many enrollment IDs.
	RetrieveEnrollmentIDs(ctx context.Context, declarations []string, sets []string, ids []string) ([]string, error)