With our PoC at
https://github.com/kevink-sq/jetty-concurrency-issue-poc/tree/575469b3453e7b09d9d60f6460cdab8117b8e773,
we noticed that there is unbounded growth of file descriptors when
running our simple load test:

```bash
seq 1 15000 | xargs -P1500 -I{} curl --unix-socket ./junix-ingress.sock http://localhost/process
```

Leading to timeouts from curl and timeout exceptions in jetty:

```
java.util.concurrent.TimeoutException: Idle timeout expired: 32422/30000 ms
        at org.eclipse.jetty.io.IdleTimeout.checkIdleTimeout(IdleTimeout.java:170)
        at org.eclipse.jetty.io.IdleTimeout.idleCheck(IdleTimeout.java:112)
        at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
        at java.base/java.lang.Thread.run(Thread.java:833)
```

This patch fixes the issue by ensuring that keys with the `OP_INVALID`
bit are still tracked in the selected key set and cancelled accordingly

Changes:
* Add invalid keys to `selectedKeysSet` in `setOpsReady` and mark as
  cancelled, ensuring that the fd is cleaned up timely
* Switch from `cancelNoRemove` to `cancel` now that the registered keys
  are modified separately in the select implementation
* Prefer `!key.isValid()` over `key.hasOpInvalid()` since the former is
  a superset of the latter
* In `implCloseSelector`, cancel the keys before clearing
  `keysRegistered` since `keys()` is backed by `keysRegistered`
* Store cancelled keys in a deque instead of a hash set, which should
  be a little more efficient
* Remove dead code

`processDeregisterQueue`, respectively

This naming convention more closely aligns with the selector
implementations in Java's standard library

head of the strings

* Place bulk of `select0` operations under `this` and
  `publicSelectedKeys` locks to reduce locking/unlocking thrash given
  frequency of locking operations
* Lock `publicSelectedKeys` instead of `selectedKeys` since that is what
  clients of AFSelector are locking on
* Eschew uses of synchronized methods in favor of locking `this` via
  synchronized blocks
* Use assertions to check lock invariants