Fix bug #179

Almost certainly the same as reported at https://github.com/Shopify/sarama/pull/199#issuecomment-63636565

Go's closure semantics are really annoying - simply stop spawning goroutines
with the wrong arguments.

Add a test (heavily based on https://gist.github.com/ORBAT/d0adcd790dff34b37b04)
to ensure this behaviour doesn't regress.

Huge thanks to Tom Eklöf for getting me all the logs etc. needed to track this
down.

@wvanbergen @ORBAT