BUG-1279: keep an insertion-sorted iterable of serializers

This will retain the order in which the AttributeSerializers have been
registered, so we can iterate over them in that order.

Change-Id: I0adbec38e79febd2c3084d1f6db65c524c2465c2
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1279: keep an insertion-sorted iterable of serializers"