Add hashmap to qpz_heap

Add hashmap to qpz_heap

This commit adds a level of indirection to the signing operations.
Instead of being intrusive, the qpz_heap will keep track of which
headers must be resigned through a hashmap.
The intent is to make dns_vecheader_t entirely self-contained. In
particular, the ownership structure between the heap and the headers is
flipped. Before, the headers would "own" the heap, now the heap owns
the header.