plugin/file: load secondary zones lazily on startup (#2944)

This fixes a long standing bug:
fixes: #1609

Load secondary zones in a go-routine; this required another mutex to
protect some fields; I think those were needded anyway because a
transfer can also happen when we're running; we just didn't have a test
for that situation.

The test had to be changed to wait for the transfer to happen at this is
async now.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/file/lookup.go

@@ -44,7 +44,9 @@ func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string)
 	// If z is a secondary zone we might not have transferred it, meaning we have
 	// all zone context setup, except the actual record. This means (for one thing) the apex
 	// is empty and we don't have a SOA record.
+	z.apexMu.RLock()
 	soa := z.Apex.SOA
+	z.apexMu.RUnlock()
 	if soa == nil {
 		return nil, nil, nil, ServerFailure
 	}