btr_cur_pessimistic_update(): Note why the externally stored columns
of a record on a latched page cannot have been purged.

trx_undo_get_undo_rec(): Clarify that the stack of versions is locked
all the way down to the purge view.

trx_undo_prev_version_build(): Set *old_vers = NULL also when the record
could have been purged already.  Add some clarifying comments.