Cache reference diffstate values (#15561)

Cache reference diffstate values (#15561)

Benchmarked with the "Set 40 panels as content" action in
BasicPerformanceTest. This is really a worst case scenario since it
doesn't do anything else than create lots of components, whereas more
common use cases would spend more time updating existing components or
executing business logic instead.

Without this patch, each action spent about 6 ms creating reference
diffstate values, making up about 20% of the total processing time. With
the patch applied, the time (including the new map lookup) was reduced
to around 0.2 ms and the total processing time was also reduced
accordingly.

Change-Id: If22a73b591b87793c78cb360bcfa8e030f003730