labeling refactor

labeling refactor

To service #6718 and #6710, the system by which columns are
given labels in a SELECT statement as well as the system that
gives them keys in a .c or .selected_columns collection have
been refactored to provide a single source of truth for
both, in constrast to the previous approach that included
similar logic repeated in slightly different ways.

Main ideas:

1. ColumnElement attributes ._label, ._anon_label, ._key_label
   are renamed to include the letters "tq", meaning
   "table-qualified" - these labels are only used when rendering
   a SELECT that has LABEL_STYLE_TABLENAME_PLUS_COL for its
   label style; as this label style is primarily legacy, the
   "tq" names should be isolated so that in a 2.0 style application
   these aren't being used at all

2. The means by which the "labels" and "proxy keys" for the elements
   of a SELECT has been centralized to a single source of truth;
   previously, the three of _generate_columns_plus_names,
   _generate_fromclause_column_proxies, and _column_naming_convention
   all had duplicated rules between them, as well as that there
   were a little bit of labeling rules in compiler._label_select_column
   as well; by this we mean that the various "anon_label" "anon_key"
   methods on ColumnElement were called by all four of these methods,
   where there were many cases where it was necessary that one method
   comes up with the same answer as another of the methods.  This
   has all been centralized into _generate_columns_plus_names
   for all the names except the "proxy key", which is generated
   by _column_naming_convention.

3. compiler._label_select_column has been rewritten to both not make
   any naming decisions nor any "proxy key" decisions, only whether
   to label or not to label; the _generate_columns_plus_names method
   gives it the information, where the proxy keys come from
   _column_naming_convention; previously, these proxy keys were matched
   based on restatement of similar (but not really the same) logic in
   two places.   The heuristics of "whether to label or not to label"
   are also reorganized to be much easier to read and understand.

4. a new method compiler._label_returning_column is added for dialects
   to use in their "generate returning columns" methods.   A
   github search reveals a small number of third party dialects also
   doing this using the prior _label_select_column method so we
   try to make sure _label_select_column continues to work the
   exact same way for that specific use case; for the "SELECT" use
   case it now needs

5. After some attempts to do it different ways, for the case where
   _proxy_key is giving us some kind of anon label, we are hard
   changing it to "_no_label" right now, as there's not currently
   a way to fully match anonymized labels from stmt.c or
   stmt.selected_columns to what will be in the result map.  The
   idea of "_no_label" is to encourage the user to use label('name')
   for columns they want to be able to target by string name that
   don't have a natural name.

Change-Id: I7a92a66f3a7e459ccf32587ac0a3c306650daf11