I just realized that using DefaultSelector isn't optimal for Connection:
It's fine for forkserver, since it's a long lived process, but for
Connection.poll(), this means that it'll use epoll or kqueue: which
means allocating a new epoll/kqueue object for each conn.poll().
That's a couple syscalls more (epoll_create()/epoll_ctl()/close()),
but most important it uses an extra FD per connection.

The attached patch uses PollSelector if available, otherwise SelectSelector.