Fix memory leak with domains

This fixes an issue where memory was slowly leaked if you were using domains with node-postgres.  I don't understand the underlying reason with the memory leak - everything I did looked like it lined up with the node.js documentation on domains.  None the less I confirmed (and was bitten in production by) a memory leak here and this code resolves it while maintaining functionality for the majority of the use cases. 

With this release I am going to unpublish the buggy releases.