BUG 2350: Cleanup binding proxy instantiation

Profiling of BGP topology provider has revealed a performance bottleneck
when instantiating proxies for list children. As it turns out around 68%
of CPU cycles is being burned in looking up the proxy class and its
contructor. This patch performs the obvious tradeoff by caching it and
exposing a utility function to subclasses.

Change-Id: Icb76f232df8223925fa802859313bbf63ee58eeb
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 8c639bd626d98b3c2c2c60067928b06cca8a94fc)