Make curation semaphore behavior idiomatic.

Idiomatic semaphore usage in Go, unless it is wrapping a concrete type,
should use anonymous empty structs (``struct{}``).  This has several
features that are worthwhile:

  1. It conveys that the object in the channel is likely used for
     resource limiting / semaphore use.  This is by idiom.

  2. Due to magic under the hood, empty structs have a width of zero,
     meaning they consume little space.  It is presumed that slices,
     channels, and other values of them can be represented specially
     with alternative optimizations.  Dmitry Vyukov has done
     investigations into improvements that can be made to the channel
     design and Go and concludes that there are already nice short
     circuiting behaviors at work with this type.

This is the first change of several that apply this type of change to
suitable places.

In this one change, we fix a bug in the previous revision, whereby a
semaphore can be acquired for curation and never released back for
subsequent work: http://goo.gl/70Y2qK.  Compare that versus the
compaction definition above.

On top of that, the use of the semaphore in the mode better supports
system shutdown idioms through the closing of channels.

Change-Id: Idb4fca310f26b73c9ec690bbdd4136180d14c32d