Add unique_constraint_name to MSSQL FK reflection
Fixed bug in MSSQL reflection where when two same-named tables in different
schemas had same-named primary key constraints, foreign key constraints
referring to one of the tables would have their columns doubled, causing
errors. Pull request courtesy Sean Dunn.
Fixes: #4228
Change-Id: I7dabaaee0944e1030048826ba39fc574b0d63031
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/457
(cherry picked from commit
ca94ea8ab583f8ab366ee5971bfc1bdd96e54cc9)