Remove `USAGE` from the `VALID_PRIVS` dict for both database and
table because it is not a valid privilege for either (and
breaks the implementation of `has_table_privilege` and
`has_database_privilege`
See http://www.postgresql.org/docs/9.0/static/sql-grant.html
Use `has_table_privileges` and `has_database_privileges`
to test whether a user already has a privilege before
granting it, or whether a user doesn't have a privilege
before revoking it.
For read-only databases, users should not change when no changes
are required.
Don't issue ALTER ROLE when role attribute flags, users password
or expiry time is not changing.
In certain cases (hashed passwords in the DB, but the password
argument is not hashed) passlib.hash is required to avoid
running ALTER ROLE.
Some places ([AWS RDS](https://forums.aws.amazon.com/thread.jspa?threadID=151248)) don't have, or don't allow, access to the `pg_authid` table. The only reason that is necessary is to check for a password change.
This flag is a workaround so passwords can only be set at creation time. It isn't as elegant as changing the password down the line, but it fixes the longstanding issue #297 that prevented this from being useful on AWS RDS.
