[SPARK-4393] Fix memory leak in ConnectionManager ACK timeout TimerTasks; use HashedWheelTimer

This patch is intended to fix a subtle memory leak in ConnectionManager's ACK timeout TimerTasks: in the old code, each TimerTask held a reference to the message being sent and a cancelled TimerTask won't necessarily be garbage-collected until it's scheduled to run, so this caused huge buildups of messages that weren't garbage collected until their timeouts expired, leading to OOMs.

This patch addresses this problem by capturing only the message ID in the TimerTask instead of the whole message, and by keeping a WeakReference to the promise in the TimerTask.  I've also modified this code to use Netty's HashedWheelTimer, whose performance characteristics should be better for this use-case.

Thanks to @cristianopris for narrowing down this issue!
why not 
```scala
val p = promiseReference.get
if (p == null) {
  ...
} else {
  ...
}
```
?