Revert "Remove now-unused SystemTypes table"

This reverts commit 71d020735bb6f8e2f46a7813fe73d2a8dbe37add. Unfortunately there are still some cases where we need to set Hydra's concurrency separately. (Ideally, Hydra would start all queued builds in parallel and let Nix take care of everything…)

Created by  Eelco Dolstra  on March 5, 2013
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