Disable auto store optimisation for now

I've seen operations like "nix-store --import" take much longer on one
system.  So default to off until I've investigated this a bit further.

src/libstore/globals.cc

@@ -43,7 +43,7 @@ Settings::Settings()
     checkRootReachability = false;
     gcKeepOutputs = false;
     gcKeepDerivations = true;
-    autoOptimiseStore = true;
+    autoOptimiseStore = false;
     envKeepDerivations = false;
 }