Have the HST use its own StringPool object instead of one shared

Because at the end of the hst config model loading, the HST does for memory efficiency:

StringPool.clear();

We cannot use the global string pool from commons.

I think best is to have the HST get a StringPool instance from commons.

I know you had this before Nour, I think that was correct. Apologies

(only don't add the finalize again)

Regards Ard