Bug 921050 - Send email in the background.

This comment should probably make some mention about whether this is persisted to the database or not/whether we wish it was or not.  Currently it won't hit the database until the operation completes (one way or the other) which means that only the finale success/error state will be persisted unless some other concurrent process decides to checkpoint the database state while we're running (note 1).  This seems okay to me as it doesn't seem like the 'sending' state is meaningful/useful when restored from disk.

note 1: When we change up the mutex implementation, this is something that seems like it could potentially happen.