Use a `MessageSendEvent`-handling fn rather than a single lopp

Use a `MessageSendEvent`-handling fn rather than a single lopp

Rather than building a single `Vec` of `MessageSendEvent`s to
handle then iterating over them, we move the body of the loop into
a lambda and run the loop twice. In some cases, this may save a
single allocation, but more importantly it sets us up for the next
commit, which needs to know from which handler the
`MessageSendEvent` it is processing came from.