Cleanup: Move MEM_ROOT allocation methods from THD to Query_arena

  inline void* alloc(size_t size) const { return alloc_root(mem_root,size); }

  inline void* calloc(size_t size) const

  {

    void *ptr;

    if (likely((ptr=alloc_root(mem_root,size))))

      bzero(ptr, size);

    return ptr;

  }

  inline char *strdup(const char *str) const

  { return strdup_root(mem_root,str); }

  inline char *strmake(const char *str, size_t size) const

  { return strmake_root(mem_root,str,size); }

  inline LEX_CSTRING strcat(const LEX_CSTRING &a, const LEX_CSTRING &b) const

  {

    char *buf= (char*)alloc(a.length + b.length + 1);

    if (unlikely(!buf))

      return null_clex_str;

    memcpy(buf, a.str, a.length);

    memcpy(buf + a.length, b.str, b.length);

    buf[a.length + b.length] = 0;

    return {buf, a.length + b.length};

  }

  inline void *memdup(const void *str, size_t size) const

  { return memdup_root(mem_root,str,size); }

  inline void *memdup_w_gap(const void *str, size_t size, size_t gap) const

  {

    void *ptr;

    if (likely((ptr= alloc_root(mem_root,size+gap))))

      memcpy(ptr,str,size);

    return ptr;

  }

  LEX_CSTRING strmake_lex_cstring(const char *str, size_t length) const

  {

    const char *tmp= strmake_root(mem_root, str, length);

    if (!tmp)

      return {0,0};

    return {tmp, length};

  }

  LEX_CSTRING strmake_lex_cstring(const LEX_CSTRING &from) const

  {

    return strmake_lex_cstring(from.str, from.length);

  }

  LEX_STRING *make_lex_string(LEX_STRING *lex_str, const char* str,

                              size_t length) const

  {

    if (!(lex_str->str= strmake_root(mem_root, str, length)))

    {

      lex_str->length= 0;

      return 0;

    }

    lex_str->length= length;

    return lex_str;

  }

  LEX_CSTRING *make_lex_string(LEX_CSTRING *lex_str, const char* str,

                               size_t length) const

  {

    if (!(lex_str->str= strmake_root(mem_root, str, length)))

    {

      lex_str->length= 0;

      return 0;

    }

    lex_str->length= length;

    return lex_str;

  }

};