CancellationToken support for Execute*

NpgsqlCommands.Execute* methods now properly support CancellationToken's protocol.
When cancellation is signalled through the token, a Postgresql cancellation message
is sent. The resulting NpgsqlError is translated to an OperationCanceledException.
Fixed some bugs

Introduced in c8d0b935d1cef429bbbb5a9a2ad258dab7bbb92d