Rework data.api.InstanceIdentifier to work on Iterables

Rework data.api.InstanceIdentifier to work on Iterables

Profiling storage-dominated workloads shows that
InstanceIdentifier.node() is dominating by taking up 16% of the CPU
cycles, with 82% of that being spent in ImmutableList$Builder.addAll().
All of these calls occur in the data change resolution path, where they
account for 53% of CPU time.

This patch removes the ImmutableList at the heart of InstanceIdentifier
with an Iterable, along with all the machinery we know and love from the
binding InstanceIdentifier.

While we are at it, perform complete house-cleaning, compacting code and
tuning it for perfomance.

Change-Id: I52884d5660086ac53647a2a39127578017449b2d
Signed-off-by: Robert Varga <rovarga@cisco.com>