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().

src/rtree.c

@@ -424,3 +424,14 @@ rtree_leaf_elm_witness_release(tsdn_t *tsdn, const rtree_t *rtree,
 	witness_unlock(tsdn, witness);
 	rtree_leaf_elm_witness_dalloc(tsdn_tsd(tsdn), witness, elm);
 }
+
+bool
+rtree_ctx_data_init(rtree_ctx_t *ctx) {
+	for (unsigned i = 0; i < RTREE_CTX_NCACHE; i++) {
+		rtree_ctx_cache_elm_t *cache = &ctx->cache[i];
+		cache->leafkey = RTREE_LEAFKEY_INVALID;
+		cache->leaf = NULL;
+	}
+
+	return false;
+}