WL#7583: Refactor strings: use of (const char *) in THD

Safe refactoring changes are needed while refactoring strings in the server in
order to minimize merge complexity.

To recap:
  - the server codebase now actively uses (char *) and LEX_STRING
    to deal with strings;
  - the reason for this is mostly legacy. In other words, even
    immutable strings are represented as (char *).
  - new strings will be mostly dealing with (const char *);
  - the idea of "safe refactoring changes" is to change (char *)
    to (const char *) where possible. Those changes are essentially
    no-change, in the sense that they don't change the behaviour.
    Because of that the changes can be pushed directly to trunk.

This WL is to change (char *) and LEX_STRING to (const char *)
and LEX_CSTRING respectively in the THD class and related functions.

This WL can also be seen as a pre-requisite for defining and providing
methodology to switch to new strings because the "safe refactoring changes" will
be a part of the methodology.

User Documentation
==================

No user-visible changes. No user documentation needed.