Reuse build products / metrics stored in the database

Previously, if the queue monitor thread encounters a build that Hydra
has previously built, it downloaded the output paths from the binary
cache, just to determine the build products and metrics. This is very
inefficient. In particular, when doing something like merging
nixpkgs:staging into nixpkgs:master, the queue monitor thread will be
locked up for a long time fetching files from S3, causing the build
farm to be mostly idle.

Of course this is entirely unnecessary, since the build
products/metrics are already in the Hydra database. So now we just
look up a previous build with the same output path, and copy the
products/metrics.

src/hydra-queue-runner/state.hh

@@ -410,6 +410,9 @@ private:
     /* Handle cancellation, deletion and priority bumps. */
     void processQueueChange(Connection & conn);
 
+    BuildOutput getBuildOutputCached(Connection & conn, nix::ref<nix::Store> destStore,
+        const nix::Derivation & drv);
+
     Step::ptr createStep(nix::ref<nix::Store> store,
         Connection & conn, Build::ptr build, const nix::Path & drvPath,
         Build::ptr referringBuild, Step::ptr referringStep, std::set<nix::Path> & finishedDrvs,