util/IntrusiveHashSet: rename insert() to insert_commit()

Clarify that the method is not a freestanding insertion method but
should only be used after insert_check().

src/RemoteTagCache.cxx

@@ -31,7 +31,7 @@ RemoteTagCache::Lookup(const std::string &uri) noexcept
 	auto [tag, value] = map.insert_check(uri);
 	if (value) {
 		auto item = new Item(*this, uri);
-		map.insert(tag, *item);
+		map.insert_commit(tag, *item);
 		waiting_list.push_back(*item);
 		lock.unlock();