postgresql.py:
- Map the metric names directly to the column names
- Turn on autocommit on the connection, or the stats wont update,
  as pg_stat_database is only updated at the start of a transcation.
- Fix a bug where only all databsae stats get mapped to the
  last database

Signed-off-by: Vasiliy Kiryanov <vasiliy.kiryanov@gmail.com>