WL#8063: True bottom-up server parser: refactoring of the LOAD statement

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

This WL is intended to refactor LOAD DATA/XML statement grammar rules in a pure bottom-up style to make it context-independent for the better maintainability and extendibility.
Functional requirements
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N/A

Non-functional requirements
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NF-1: No performance degradation is expected.
User Documentation
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No user-visible changes. No documentation required. 
New classes
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Class name   | Description
-------------|------------------------------------
PT_load      | Parse tree root for `LOAD DATA/XML`
Sql_cmd_load | Command class for `LOAD DATA/XML`


Removal of unneeded "global" variables
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* LEX::exchange
* LEX::load_field_list
* LEX::load_update_list
* LEX::load_value_list
* LEX::load_set_str_list
* LEX::local_file