Backport - Jcr quartz integration allows jobs to be run concurrently on different cluster nodes

JcrJobStore#aquireNextTrigger tries to prevent concurrency by testing for the existence of a property. But the following test fails on our current JR version:

@Test
public void multiSessionConflictIsNotResolved() throws Exception

{ final Node test = session.getRootNode().addNode("test"); test.setProperty("aap", "noot"); session.save(); session.refresh(false); session.getNode("/test").getProperty("aap").remove(); Session other = session.impersonate(new SimpleCredentials(null, "".toCharArray())); other.getNode("/test").getProperty("aap").remove(); other.save(); session.save(); fail("Session save should have failed"); }

Non conflicting changes from other sessions are merged. The behavior used to be that an exception was thrown, which resulted in only one cluster node executing a particular job.
We need to use JCR locking instead.