improve closed connection handling

[GertSallaerts]

Related to my previous PR (#137).

• Not sure why it seemed to work, but the _handleConnectionTerminated handler never really worked because it wasn't bound correctly and 'this.onerror is always undefined. So I bound the function.
• Upon reflection, I don't think that the server closing a connection should be considered as an error, the connection is simply not open anymore for whatever reason. Once you try to open a new connection, you'll get a more accurate error message like the server not being reachable anymore for example or no error is there is a failover setup configured, ... So instead of throwing an error, I close the connection.
• The pool did not call the _destroy method on resources that are detected as invalid during acquisition, so they weren't being cleaned up correctly.

[pontusmelke]

Awesome, thanks! I am currently looking into this. I think I will end up putting the onerror call back.