Close AsyncServerSocket sockets in reverse order

Close AsyncServerSocket sockets in reverse order

Summary:
When a large number of processes concurrently bind and close `AsyncServerSocket`s with `port=0` (for example in tests) binding the IPv4 socket on the same port bound with the IPv6 socket can currently fail, because sockets are closed in the same order as they are opened, which makes the following scenario possible:

```
P0: close IPv6 port
P1: open IPv6 port (succeed)
P1: open IPv4 port (FAIL)
P0: close IPv4 port
```

This diff reverses the closing order, and also fixes a couple of outdated comments.

Reviewed By: philippv

Differential Revision: D2795920

fb-gh-sync-id: 0b5157a56dfed84aed4a590c103050a2727847b3