archive blocks before overwriting them

lib/storage/block.js

@@ -72,9 +72,16 @@ Block.write = function (Env, publicKey, buffer, _cb) {
             w.abort();
             cb(err);
         }));
+    }).nThen(function (w) {
+        Block.archive(Env, publicKey, w(function (/* err */) {
+    /*
+        we proceed even if there are errors.
+        it might be ENOENT (there is no file to archive)
+        or EACCES (bad filesystem permissions for the existing archived block?)
+        or lots of other things, none of which justify preventing the write
+    */
+        }));
     }).nThen(function () {
-        // XXX BLOCK check whether this overwrites a block
-        // XXX archive the old one if so
         Fs.writeFile(path, buffer, { encoding: 'binary' }, cb);
     });
 };