The `static_rtree.hpp` header included `<booost/thread.hpp>` without using
anything from this header.
Removing it showed why:
the unit test for the rtree no longer built, since it was missing symbols
for Boost's `hash_combine`, used in the unit test.
Instead of relying on `<boost/thread.hpp>` including the proper header
for `hash_combine` by chance that we only use in the unit test, do the
following:
- remove `<boost/thread.hpp>` from the rtree implementation
- add `<boost/functional/hash.hpp>` to the rtree unit test
As always, include what you use.
