producer: enable retry-backoff

Wait a bit (250ms by default) for the cluster to elect a new leader before
processing message retries, since otherwise chances are we'll just get stale
metadata from the broker we ask and the client won't know enough to back off on
its own (because stale metadata won't contain any errors).

Fixes #229.

@wvanbergen @yagnik @drdee @mkobetic @snormore