optimiseStore(): Use a content-addressed file store in /nix/store/.links

optimiseStore() now creates persistent, content-addressed hard links
in /nix/store/.links.  For instance, if it encounters a file P with
hash H, it will create a hard link

  P' = /nix/store/.link/<H>

to P if P' doesn't already exist; if P' exist, then P is replaced by a
hard link to P'.  This is better than the previous in-memory map,
because it had the tendency to unnecessarily replace hard links with a
hard link to whatever happened to be the first file with a given hash
it encountered.  It also allows on-the-fly, incremental optimisation.

src/libstore/local-store.hh

@@ -167,7 +167,7 @@ public:
 
     /* Optimise the disk space usage of the Nix store by hard-linking
        files with the same contents. */
-    void optimiseStore(bool dryRun, OptimiseStats & stats);
+    void optimiseStore(OptimiseStats & stats);
 
     /* Check the integrity of the Nix store. */
     void verifyStore(bool checkContents);