Jetty patch to avoid hanging threads

[xiaochen-db]

The investigation of https://databricks.atlassian.net/browse/PROD-35631 suggested a jetty websocket's sendString() can lead to the calling thread hanging on blockingWrite (See jetty#2061 for the OSS discussion).

This PR implemented the workaround that's suggested by the OSS community (jetty#2061 (comment)) to avoid hanging threads like

"WebappServiceThreadPool-197" #197 prio=5 os_prio=0 cpu=40130.40ms elapsed=88215.54s tid=0x00007fcc2f4f1800 nid=0xe5 waiting on condition  [0x00007fca389c9000]
   java.lang.Thread.State: WAITING (parking)
	at jdk.internal.misc.Unsafe.park(java.base@11.0.7/Native Method)
	- parking to wait for  <0x00000003a16421b8> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
	at java.util.concurrent.locks.LockSupport.park(java.base@11.0.7/LockSupport.java:194)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@11.0.7/AbstractQueuedSynchronizer.java:2081)
	at org.eclipse.jetty.util.SharedBlockingCallback$Blocker.block(SharedBlockingCallback.java:203)
	at org.eclipse.jetty.websocket.common.BlockingWriteCallback$WriteBlocker.block(BlockingWriteCallback.java:82)
	at org.eclipse.jetty.websocket.common.WebSocketRemoteEndpoint.blockingWrite(WebSocketRemoteEndpoint.java:107)
	at org.eclipse.jetty.websocket.common.WebSocketRemoteEndpoint.sendString(WebSocketRemoteEndpoint.java:393)
	at com.databricks.webapp.comm.DriverWebSocket.$anonfun$sendDNCMessage$4(DriverConnection.scala:618)
	at com.databricks.webapp.comm.DriverWebSocket.$anonfun$sendDNCMessage$4$adapted(DriverConnection.scala:618)
	at com.databricks.webapp.comm.DriverWebSocket$$Lambda$7464/0x0000000842ba3040.apply(Unknown Source)
	at scala.Option.foreach(Option.scala:407)
	at com.databricks.webapp.comm.DriverWebSocket.sendDNCMessage(DriverConnection.scala:618)
	at com.databricks.webapp.comm.DriverConnection.send(DriverConnection.scala:295)
	- locked <0x00000003d1fbdc38> (a com.databricks.webapp.comm.DriverConnection)
	at com.databricks.webapp.comm.DirectNotebookChannelManagerImpl.$anonfun$handleMsgToDriver$1(DirectNotebookChannelManager.scala:403)
	at com.databricks.webapp.comm.DirectNotebookChannelManagerImpl$$Lambda$6641/0x000000084285fc40.apply$mcV$sp(Unknown Source)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at com.databricks.logging.UsageLogging.$anonfun$recordOperation$1(UsageLogging.scala:562)
	at com.databricks.logging.UsageLogging$$Lambda$627/0x0000000840bd7c40.apply(Unknown Source)

Please note that branch origin/jetty-patch-to-avoid-hanging-threads is directly checked out from tag jetty-9.3.30.v20211001 (used by webapp):

* 148ea77050 - (HEAD -> jetty-patch-to-avoid-hanging-threads, tag: jetty-9.3.30.v20211001, origin/jetty-patch-to-avoid-hanging-threads) Updating to version 9.3.30.v20211001 (1 year, 7 months ago) <Joakim Erdfelt>

We plan to publish this change to origin/jetty-patch-to-avoid-hanging-threads as a new version, and then apply it to webapp repo.