[Twisted-Python] reflector init

[Jonathan M. Lange]

The initialisers for reflectors seem to do some interesting stuff. The
SQLReflector in particular does some stuff that requires a callback.
(see twisted.enterprise.*reflector)

Shouldn't this be handled using a Deferred? It's extremely frustrating 
not being able to add an errback.

Suggestions include having a populatedDeferred as a member (rather than
populatedCallback), or moving the populate stuff into a separate method 
that returns a Deferred.

Thoughts?

jml