WL#8065: True bottom-up server parser: refactoring of the EXPLAIN statement

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

Note: this WL is almost implemented in the context of eralier WLs.
The rest of work: EXPLAIN/DESCRIBE TABLE and EXPLAIN FOR CONNECTION.

This WL is intended to refactor EXPLAIN (and its aliases:
DESCRIBE/DESC <statement>) statement-related parser grammar rules in a pure
bottom-up style to make it (grammar) context-independent for the better
maintainability and extendibility.
Functional requirements
-----------------------

N/A

Non-functional requirements
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NF-1: No performance degradation is expected.
Goals
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This WL is intended to refactor the grammar of the `EXPLAIN/DESCRIBE/DESC  SELECT/INSERT/REPLACE/UPDATE/DELETE/FOR CONNECTION` statement in a pure context-independent bottom-up style for the better maintainability and extendibility.

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".

User Documentation
------------------

No user-visible changes. No documentation required. 
New parse tree node classes
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Class name                  | Related syntax
----------------------------|------------------
PT_explain_for_connection   | `FOR CONNECTION <query-id>`
PT_explain                  | `EXPLAIN/DESCRIBE/DESC SELECT/INSERT/REPLACE/UPDATE/DELETE/FOR CONNECTION ...`


Removal of unneeded "global" variables and constants
----------------------------------------------------

* LEX::describe
* DESCRIBE_NONE and DESCRIBE_NORMAL