Clear any immutable bits in the Nix store

Doing this once makes subsequent operations like garbage collecting
more efficient since we don't have to call makeMutable() first.

src/libstore/local-store.hh

@@ -17,8 +17,8 @@ namespace nix {
 /* Nix store and database schema version.  Version 1 (or 0) was Nix <=
    0.7.  Version 2 was Nix 0.8 and 0.9.  Version 3 is Nix 0.10.
    Version 4 is Nix 0.11.  Version 5 is Nix 0.12-0.16.  Version 6 is
-   Nix 1.0. */
-const int nixSchemaVersion = 6;
+   Nix 1.0.  Version 7 is Nix 1.3. */
+const int nixSchemaVersion = 7;
 
 
 extern string drvsLogDir;
@@ -265,6 +265,7 @@ private:
     void updatePathInfo(const ValidPathInfo & info);
 
     void upgradeStore6();
+    void upgradeStore7();
     PathSet queryValidPathsOld();
     ValidPathInfo queryPathInfoOld(const Path & path);