Kill all queued background jobs when closing the database

This is not urgent, but removing all jobs related to the closing database is better.

If the jobs are not killed, we'll encounter Java exception:

!ENTRY org.eclipse.e4.ui.workbench 4 0 2024-06-13 18:35:17.365
!MESSAGE Internal Error
!STACK 0
java.lang.IllegalStateException: Need an underlying widget to be able to set the input.(Has the widget been disposed?)
	at org.eclipse.jface.viewers.StructuredViewer.setInput(StructuredViewer.java:1627)
	at edu.rice.cs.hpctraceviewer.ui.callstack.CallStackViewer.lambda$1(CallStackViewer.java:230)
	at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:40)
	at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:132)
	at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4368)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3991)
	at org.eclipse.swt.widgets.Display.release(Display.java:4075)
	at org.eclipse.swt.graphics.Device.dispose(Device.java:241)
	at org.eclipse.e4.ui.internal.workbench.swt.E4Application.start(E4Application.java:183)
	at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:203)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:136)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:402)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:255)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:659)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:596)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1467)
	at org.eclipse.equinox.launcher.Main.main(Main.java:1440)