1. TAL-SH C++ implementation has assert with side effects;
2. talsh::Tensor::impl::~impl dtor currently expects write_task_ member to be NULL: Wrong:
   a. If Tensor.impl->write_task_ != NULL, simply check task completion and remove tensor pointer
      from inside the TensorTask (stored pointers to participating tensors).
   b. TensorTask dtor must also nullify write_task_ pointers in all participating tensors.
      Use double friendship.