Improve use of bson_init_static

We call bson_init_static repeatedly and redundantly (e.g. on every element access).
We should call bson_init_static once on view creation and cache the result.

Update: we can't cache a bson_t because it uses internal references and it therefore non-copyable without dangling pointers. We've proposed CDRIVER-2064, which would allow us to construct bson_iter_t structures directly directly from the view buffer.