Make the garbage collector more concurrent

Make the garbage collector more concurrent by deleting valid paths
outside the region where we're holding the global GC lock.  This
should greatly reduce the time during which new builds are blocked,
since the deletion accounts for the vast majority of the time spent in
the GC.

To ensure that this is safe, the valid paths are invalidated and
renamed to some arbitrary path while we're holding the lock.  This
ensures that we when we finally delete the path, it's not a (newly)
valid or locked path.

src/libstore/local-store.hh

@@ -164,10 +164,6 @@ public:
 
     void collectGarbage(const GCOptions & options, GCResults & results);
 
-    /* Delete a path from the Nix store. */
-    void deleteFromStore(const Path & path, unsigned long long & bytesFreed,
-        unsigned long long & blocksFreed);
-    
     /* Optimise the disk space usage of the Nix store by hard-linking
        files with the same contents. */
     void optimiseStore(bool dryRun, OptimiseStats & stats);
@@ -238,6 +234,9 @@ private:
 
     void invalidatePath(const Path & path);
 
+    /* Delete a path from the Nix store. */
+    void invalidatePathChecked(const Path & path);
+    
     void verifyPath(const Path & path, const PathSet & store,
         PathSet & done, PathSet & validPaths);
 
@@ -249,6 +248,8 @@ private:
 
     struct GCState;
 
+    void deleteGarbage(GCState & state, const Path & path);
+    
     bool tryToDelete(GCState & state, const Path & path);
     
     bool isActiveTempFile(const GCState & state,