(SUBSTRING inside a stored function works too slow).

Background:
  - THD classes derives from Query_arena, thus inherits the 'state'
    attribute and related operations (is_stmt_prepare() & co).

  - Although these operations are available in THD, they must not
    be used. THD has its own attribute to point to the active
    Query_arena -- stmt_arena.

  - So, instead of using thd->is_stmt_prepare(),
    thd->stmt_arena->is_stmt_prepare() must be used. This was the root
    cause of Bug 60025.

This patch enforces the proper way of calling those operations.
is_stmt_prepare() & co are declared as private operations
in THD (thus, they are hidden from being called on THD instance).

The patch tries to minimize changes in 5.5.