Made get_indexes() consistent across backends.

Fixed #15933, #18082 -- the get_indexes() method introspection was
done inconsitently depending on the backend. For example SQLite
included all the columns in the table in the returned dictionary,
while MySQL introspected also multicolumn indexes.

All backends return now consistenly only single-column indexes.

Thanks to andi for the MySQL report, and ikelly for comments on
Oracle's get_indexes() changes.