BUG-1275: optimized set instantiation ChoiceNodeBaseSerializer

The hashset used in getRealSchemasForAugment() can be initialized when
allocated, potentially saving some reallocation.

Change-Id: Ib848074716f5f4de108e6b348f0fed2f20bc6dcf
Signed-off-by: Robert Varga <rovarga@cisco.com>