use connection.closed, cursor.closed for disconnect detection when possible

[Mike Bayer]

libpq offers different messages based on locale so our approach does not scale. Need to remove the implicit "invalidate" in pool.py in order to have connection/cursor available in is_disconnect()

[Mike Bayer]

so much for that idea:

>>> conn = psycopg2.connect(user='scott', password='tiger', database='test')
>>> conn.closed
0
>>> conn.execute("select 1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'psycopg2._psycopg.connection' object has no attribute 'execute'
>>> cursor = conn.cursor()
>>> cursor.execute("select 1")

### <<< stop the pg database here >>> ###

>>> conn.closed
0
>>> cursor.execute("select 1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
psycopg2.OperationalError: server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.

>>> conn.closed
0
>>> cursor.closed
False
>>>

[Mike Bayer]

Removing milestone: 0.7.0 (automated comment)