IllegalStateException: Response is committed

IRC Discussion:

```
18:55:49 lkiesow | Does anyone else get a lot of │
18:55:50 lkiesow | (HttpChannel:396) - /admin-ng/event/events.json?filter=status:EVENTS.EVENTS.STATUS.RECORDING&limit=1 │
18:55:51 lkiesow | java.lang.IllegalStateException: Response is committed │
19:07:00 AndChat|190896 | I do │
19:07:04 AndChat|190896 | All the time │
19:07:14 * | AndChat|190896 is greg_logan │
19:07:53 AndChat|190896 | HttpChannel throws that exception for most admin ui requests in my experience. │
19:08:12 AndChat|190896 | No one else was complaining so I assumed it was just on my end and hadn't tracked it down yet │
19:14:27 lkiesow | I did notice it only in the last couple of days │
19:15:15 lkiesow | From when it occurs, I am wondering if it's caused by a dropped tcp connection (Opencast tries to send data but browser is already somewhere else) │
19:15:40 lkiesow | because it's mainly the counter requests if I do a page reload │
19:16:14 lkiesow | I guess we see these errors now since Rubencino made change to the CleanSessionFilter │
19:16:25 lkiesow | which before ate all errors │
19:17:10 lkiesow | But all that is just a guess and I do not have time to investigate │
19:17:18 AndChat|190896 | It's been there for a while for me, but that sounds like a reasonable guess. │
19:17:26 * | Rubencino likes to be mentioned for a good cause │
19:17:43 AndChat|190896 | I haven't had time either. Can you file an Rb for this Lars? │
19:17:59 AndChat|190896 | This should be fixed prior to 3.0 if others are seeing it
```

Error:

```
2017-05-02 18:59:49,858 | WARN | (HttpChannel:396) - /admin-ng/event/events.json?filter=status:EVENTS.EVENTS.STATUS.PROCESSING&limit=1
java.lang.IllegalStateException: Response is committed
at org.eclipse.jetty.server.Request.getSession(Request.java:1374)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.server.Request.getSession(Request.java:1352)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.opencastproject.kernel.rest.CleanSessionsFilter.doFilter(CleanSessionsFilter.java:97)[103:matterhorn-kernel:4.0.0.SNAPSHOT]
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)[292:org.eclipse.jetty.servlet:9.2.19.v20160908]
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)[292:org.eclipse.jetty.servlet:9.2.19.v20160908]
at org.ops4j.pax.web.service.jetty.internal.HttpServiceServletHandler.doHandle(HttpServiceServletHandler.java:71)[314:org.ops4j.pax.web.pax-web-jetty:4.3.0]
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)[290:org.eclipse.jetty.security:9.2.19.v20160908]
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.ops4j.pax.web.service.jetty.internal.HttpServiceContext.doHandle(HttpServiceContext.java:287)[314:org.ops4j.pax.web.pax-web-jetty:4.3.0]
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)[292:org.eclipse.jetty.servlet:9.2.19.v20160908]
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.ops4j.pax.web.service.jetty.internal.JettyServerHandlerCollection.handle(JettyServerHandlerCollection.java:80)[314:org.ops4j.pax.web.pax-web-jetty:4.3.0]
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.server.Server.handle(Server.java:499)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)[291:org.eclipse.jetty.server:9.2.19.v20160908]
at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)[283:org.eclipse.jetty.io:9.2.19.v20160908]
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)[294:org.eclipse.jetty.util:9.2.19.v20160908]
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)[294:org.eclipse.jetty.util:9.2.19.v20160908]
at java.lang.Thread.run(Thread.java:748)[:1.8.0_131]
2017-05-02 18:59:49,858 | WARN | (HttpChannel:482) - Could not send response error 500: java.lang.IllegalStateException: Response is committed
```