Don't rely on string col name in adapt_to_context

Don't rely on string col name in adapt_to_context

fixed an issue where even though the method claims to be
matching up columns positionally, it was failing on that by
looking in "keymap" based on string name.

Adds a new member to the _keymap recs MD_RESULT_MAP_INDEX
so that we can efficiently link from the generated keymap
back to the compiled._result_columns structure without
any ambiguity.

Fixes: #5559
Change-Id: Ie2fa9165c16625ef860ffac1190e00575e96761f