Refactor SchemaConstructorGen to be reused

Rather than repeat this code for every backend, allow the backend to
reuse our existing code and just initialize any state within the
Schema implementation's constructor, and not in the SchemaFactory
open() method.

Change-Id: Ie4509c61d85cf38fdb1159647bbc2dbd5b85f5d5
Signed-off-by: Shawn O. Pearce <sop@google.com>
6 files changed