[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2780: use OneShotObserverList on magic-folder branch

[Tahoe-LAFS]

#2780: use OneShotObserverList on magic-folder branch
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     Reporter:  warner  |      Owner:  daira
         Type:  task    |     Status:  new
     Priority:  normal  |  Milestone:  undecided
    Component:  code    |    Version:  1.11.0
   Resolution:          |   Keywords:
Launchpad Bug:          |
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Comment (by meejah):

 This is already in master, but named a little differently than here
 (`when_connected_enough()`).

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2780#comment:1>
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