Optimize DataObjectCodecContext memory footprint

Optimize DataObjectCodecContext memory footprint

Examining heap dumps while working on MDSAL-442/MDSAL-443 revealed
a slight inefficiency in DataObjectCodecContext: we are forcing
byStreamClass's entrySet to be instantiated for the purposes of
copying it into byBindingArgClassBuilder.

This patch corrects the mistake by using byStreamClassBuilder
as the source of copied entries -- which holds the same content,
but we will be throwing it away.

Furthermore byStreamClass and byBindingArgClass are only distinct
when we have a choice child, hence we also run comparison on the
two builders and reuse byStreamClass if they are equal.

Change-Id: Ie58367f7cc899d49da7f78cd0d4e70fbb064ae99
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>