[MERGE] Cleanups for fallback repos and stacking

[John Arbash Meinel]

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As part of trying to get GC to work with stacking, Andrew and I ran into
some bugs with RemoteRepository and fallback repos.


This patch is mostly about clarifying the contracts of locking and
fallback repositories.

The final statement we came up with for fallback repositories was:
1) When transitioning from unlocked => locked, a repo will lock its
   fallback repos
2) When transitioning from locked => unlocked, a repo will unlock its
   fallback repos
3) If self.is_locked(), add_fallback_repository() will lock the fallback
   repo, which will match (2) when self is finally unlocked.
Callers of add_fallback_repository() maintain their responsibility to
match every lock that they perform with a corresponding unlock.

John
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