Optimize binding InstanceIdentifier methods

Optimize binding InstanceIdentifier methods

Openflow traces show some overhead from UnmodifiableIterable. As it
turns out, we are instantiating them needlessly for internal iteration.
This stems from accessing getPathArguments() instead of pathArguments,
which are exposed precisely for this optimization.

Make sure to access pathArguments directly, eliminating this overhead.

Change-Id: I3ef053baa35c80f555b6b9f0b8813692236afa48
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 96639b4fd02beb3590f62fdfc096936e71ad4a32)