consumer: expose message Source

open for discussion - this is one (simple) way to expose the source
nsqd that delivered the message.

in my case, I used this to remove a head-of-line blocking issue
where a single bad actor was preventing an application from making
progress.
It *is* the `nsqd` source (i.e. the `nsqd` connection  that delivered the message)

Naming, perhaps?