Pre-requisite patch: Get control over THD::query_string.

Encapsulate access to THD::query_string:
- Make it a private member of Statement, with protected getters/setters in Statement.
- Make a single public getter and setter in THD.
- Change getter to return const.
- Rewrite QC key logic so that it doesn't have to write to the query_string buffer.
- Change Prepared_statement code to use member functions which can use the
  protected getter/setter in Statement.
- Currently one const_cast is currently required in Lex_input_stream::reset
  as the lexer in one special case directly patches the query string.