Remove parallel write from `drainRoot`.
Remove parallel write from `drainRoot`.

This particular parallelism will be rarely invoked and when it does, if
it ever raises an error, it will fire the callback twice. It is a
pointless use of parallelism.

Closes #293.

cesarmarinhorj/strata,bigeasy/strata,bigeasy/strata,cesarmarinhorj/strata