double _got_introducer if main_tab isn't started before IntroducerClient added

[meejah]

If you do this:

• create main_tub
• create IntroducerClient?
• add IC as client of main_tub (setServiceParent(main_tub))
• main_tub.startService()

...then you'll get "got introducer version" twice in the IC. If you move the "startService" up to before the setServiceParent, you only get one notification. Is there a race in Foolscap (foolscap.pb.Tub calls startConnecting on Reconnectors but sometimes does so twice). Could be due to "when does the reactor start" too (i.e. some of the startConnecting calls are in eventually()