Fix build under clang and some warnings

Build problems fixed:
- void* pointer arithmetic which is a GNU extension (works as byte*)
- "variable size object may not be initialized" which is OK under GCC
- nullptr_t related error in json11 (it lacks 'operator <' in clang)

Warnings fixed:
- empty nested struct initializer { 0 } replaced by {}
- removed several unused lambda captures

src/blockstore_impl.cpp

@@ -372,7 +372,7 @@ void blockstore_impl_t::enqueue_op(blockstore_op_t *op)
                     };
                 }
                 unstable_writes.clear();
-                op->callback = [this, old_callback](blockstore_op_t *op)
+                op->callback = [old_callback](blockstore_op_t *op)
                 {
                     obj_ver_id *vers = (obj_ver_id*)op->buf;
                     delete[] vers;