- replaced all self.__connection.is_valid with

- replaced all self.__connection.is_valid with
"safe" self._connection_is_valid.
- Fixed bug in Connection whereby if a "disconnect"
event occurred in the "initialize" phase of the
first connection pool connect, an AttributeError
would be raised when the Connection would attempt
to invalidate the DBAPI connection.  [ticket:1894]
- Connection.invalidate() can be called more than
once and subsequent calls do nothing.