Improved logging abstraction

This also gets rid of --log-type, since the nested log type isn't
useful in a multi-threaded situation, and nobody cares about the
"pretty" log type.

src/libstore/optimise-store.cc

@@ -228,7 +228,7 @@ void LocalStore::optimiseStore(OptimiseStats & stats)
     for (auto & i : paths) {
         addTempRoot(i);
         if (!isValidPath(i)) continue; /* path was GC'ed, probably */
-        startNest(nest, lvlChatty, format("hashing files in ‘%1%’") % i);
+        Activity act(*logger, lvlChatty, format("hashing files in ‘%1%’") % i);
         optimisePath_(stats, i, inodeHash);
     }
 }