Refactor insert/materialize node schemas.

The routine nvalueFromSQLDefaultType allocates its variable-length data in the long-term pool.  I really want it to allocate in the temporary thread-local pool.

To address this, I've added a new parameter (that accepts an enum value) so the caller can specify either USE_LONG_TERM_STORAGE or USE_TEMP_STORAGE.  Does this fit in ok with VoltDB C++ style?