Pool freezes when connection is interrupted

[cdoolin]

Connection.close only sets _conn = None, it does not update _connected. This causes a Pool to break when a Connection has been closed manually or if a connection error occurs causing the connection to be closed.

[waketzheng]

Should be:

if conn.connected and not conn.closed:

And would be better to add some tests

[robert-lore]

This is something we've struggled with as well. Would be nice to have a solution.