Additional listen poller cleanups

* Use sync/atomic and declare counters explicitly
* Don't differentiate the parse errors, just count them all
* s/stats/_meta_/ to help ensure we won't conflict
* Don't make quite so many new fmt.Errorf's
Merge branch 'revisit' of https://github.com/heroku/shh into revisit

Conflicts:
	listen_poller.go
	listen_poller_test.go