C++11 defines the singly-linked std::forward_list. Prefer it to
the doubly-linked std::list in cases where we dot really need it.
Also, clean up some code.

dict_index_remove_from_v_col_list(): Remove.
Obsoleted by dict_index_t::detach_columns().

There is no std::forward_list::push_back(). Use push_front() instead.
The ordering does not really matter.

dict_v_col_t::n_v_indexes: Added. There is no std::forward_list::size(),
and trx_undo_log_v_idx() needs to know the size.

rtr_info_track_t::rtr_active: Encapsulate. There really was no justification
for the pointer indirection.