Add init function support to tsd members.

This will facilitate embedding tcache into tsd, which will require proper
initialization cannot be done via the static initializer.  Make tsd->rtree_ctx
to be initialized via rtree_ctx_data_init().

include/jemalloc/internal/rtree_externs.h

@@ -43,5 +43,6 @@ void rtree_leaf_elm_witness_access(tsdn_t *tsdn, const rtree_t *rtree,
     const rtree_leaf_elm_t *elm);
 void rtree_leaf_elm_witness_release(tsdn_t *tsdn, const rtree_t *rtree,
     const rtree_leaf_elm_t *elm);
+bool rtree_ctx_data_init(rtree_ctx_t *ctx);
 
 #endif /* JEMALLOC_INTERNAL_RTREE_EXTERNS_H */