#908 rmda- add explicit isByteCopyable trait for NVCC quirk

- RdmaHandle is used in serialization. However it is neither a primitive,
  nor is it marked as byte-copyable.

  This type is appears to be forcing serialization in VT with NVCC,
  and the rules for 'is byte copyable' in VT might need to be unified
  back with checkpoint.

src/vt/rdmahandle/manager.impl.h

@@ -53,6 +53,9 @@ namespace vt { namespace rdma {
 namespace impl {
 
 struct HandleData {
+  // #949. NVCC shim: Ensure type is compatible with serialization.
+  using isByteCopyable = std::true_type;
+
   HandleData();
   HandleData(HandleKey in_key, std::size_t in_size, int in_count)
     : key_(in_key),