[#644] Support keepalive during listing

Send whitespaces every time as new object in list is ready
to prevent client from context cancelling.

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/handler/handlers_test.go

@@ -154,6 +154,10 @@ func (c *configMock) TLSTerminationHeader() string {
 	return c.tlsTerminationHeader
 }
 
+func (c *configMock) ListingKeepaliveThrottle() time.Duration {
+	return 0
+}
+
 func (c *configMock) putLocationConstraint(constraint string) {
 	c.placementPolicies[constraint] = c.defaultPolicy
 }