Consider adding a lock to `RTree`

Libspatialindex is not threadsafe, so all operations (even queries as I understand) need to be done under a lock.

The way you would do this is by adding a `lock` field to the struct, providing it a `ReentrantLock` on construction, and then calling the C-API only under a
```julia
lock(index.lock)
try
	# call CAPI here
finally
	unlock(index.lock)
end
```