Generalize uq -> uix dedupe logic

Generalize uq -> uix dedupe logic

Adjusted the logic originally added for :ticket:`276` that detects MySQL
unique constraints which are actually unique indexes to be generalized
for any dialect that has this behavior, for SQLAlchemy version 1.0 and
greater.  This is to allow for upcoming SQLAlchemy support for unique
constraint reflection for Oracle, which also has no dedicated concept of
"unique constraint" and instead establishes a unique index.

Change-Id: Ie5770aba36005ec8618bdc18bc4633413d37fc16