PostgreSQL now reflects per-column sort order on indexes.

PostgreSQL now reflects per-column sort order on indexes.

Added support for column sorting flags when reflecting indexes for
PostgreSQL, including ASC, DESC, NULLSFIRST, NULLSLAST.  Also adds this
facility to the reflection system in general which can be applied to other
dialects in future releases.  Pull request courtesy Eli Collins.

Fixes: #4717
Closes: #4725
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4725
Pull-request-sha: 3cbb067bd46776fdb125553ba0ac192cb45d060c

Change-Id: I8b0617d68580cfe4ff79d758a077263f33e852c2
(cherry picked from commit b4be7ceb86baeb8e1db4de38911a8c9e7acdd532)