Label the reaper thread

Hi Bryan,

At Erudify we're doing some GHC event analysis work (using [ghc-events-analyze](http://www.well-typed.com/blog/86)). For that it's important that all threads in our program have a descriptive label. So I would like to push the Haskell community in the direction of labeling all internally created threads with the standard label: `<package-name>: <description>`.

This patch labels the resource-pool's "reaper" thread in that way.