WL#7203: True bottom-up server parser: refactoring of the SET statement

This WL is intended to refactor SET statement-related parser
grammar rules in a pure bottom-up style to make it (grammar) context-independent
for the better maintainability and extendability at the first place.

This WL uses the framework and the methodology of the refactoring from the
WL#7199 "True bottom-up server parser: common framework for the refactoring".

The WL depends on the similar WL#7200 for the SELECT statement since that WL
covers the refactoring of "expr", "subselect" etc common rules.

No functional requirements.

Non-functional requirements:
* no query result changes;
* minor error message output changes (more specific position in the query);
* no performance degradation is expected (some additional overhead is expected
due to added allocation (cheap) and processing of parse nodes, however the
parser rules become simpler and should compensate for other degradation. Also we
eliminate a number of _current_thd() function calls -- that seems profitable.)

See WL#7199 for common detail.

The target for this WL is the "set" grammar rule.

After the refactoring the statement entry for SET clause in the grammar calls
the PT_set::contextualize() function at the top level:

statement:
...
  | set                   { CONTEXTUALIZE($1); }


The WL introduces a number of parse tree node classes:

------
PT_set
------
This class represents top-level parse tree node for the whole SET statement.

--------------------------------------------------------------------
PT_start_option_value_list_no_type, PT_start_option_value_list_type,
PT_start_option_value_list_transaction,
PT_option_value_no_option_type_password and
PT_option_value_no_option_type_password_for
--------------------------------------------------------------------
These classes represent parse tree nodes for the informative part of the SET
clause.
* PT_start_option_value_list_no_type and PT_start_option_value_list_type:
  comma-separated lists of system/user variable assignments, without or with
  a GLOBAL/LOCAL/SESSION option.
  The first node class also may hold "SET CHARSET" and "SET NAMES" charset
  manipulations (needs further grammar polishing).

* PT_start_option_value_list_transaction: transaction control stuff;

* PT_option_value_no_option_type_password and
  PT_option_value_no_option_type_password_for: user password management.

-------------------------------------------
PT_option_value_no_option_type_internal,
PT_option_value_no_option_type_user_var and
PT_option_value_no_option_type_sys_var
-------------------------------------------
Internal/user/system variable assignments.

-------------------------------------------
PT_option_value_no_option_type_charset,
PT_option_value_no_option_type_names,
PT_option_value_no_option_type_names_charset
-------------------------------------------
"SET CHARSET" and "SET NAMES" charset manipulations.
Note: PT_option_value_no_option_type_names node always contextualizes with
some syntax error (the error message text historically depends on SET NAMES
expression parsing).

------------------
PT_isolation_level
------------------
A parse tree node to save transaction isolation level type (READ UNCOMMITTED,
REPEATABLE etc.)

--------------------------
PT_transaction_access_mode
--------------------------
A parse tree node to save transaction access mode (READ ONLY/READ WRITE).

------------------------------
PT_transaction_characteristics
------------------------------
A node to link PT_isolation_level and PT_transaction_access_mode nodes
together.