Fix backend protocol version fallback bug

NpgsqlNetworkStream.Dispose() no longer tries to close its Connector unless the Connector has been explicitly attached to it.
Save the NpgsqlNetworkStream on Connector so it can be attached after successful startup.
Cleaned up CloseState.Open().
Streams

Fix typo of stream name.
Eliminate need for explicitly creating a delegate.
Change Connector.Stream type to BufferedStream, which is what it always is.
Merge pull request #83 from glenebob/pv_fallback_bugfix

Fix backend protocol version fallback bug