Lower SchemaPath#getPath() memory overhead

Lower SchemaPath#getPath() memory overhead

Acquiring forward path involves performing a reverse() on an
ImmutableList -- which results in an proxy object being created.
Each such object costs 24 bytes, leading to ~120K wasted memory on a
freshly-booted up BGP+OF instance.

Eliminate this overhead by instantiating a temporary ArrayList and
copying its reverse view. This way only a single object is retained.

Change-Id: If3b193e84f0f7ffc17997360ae5b02669c119321
Signed-off-by: Robert Varga <rovarga@cisco.com>