occasional CacheInvalidationIT failure

[Julian Reschke]

testCacheInvalidationHierarchical(org.apache.jackrabbit.oak.plugins.document.mongo.CacheInvalidationIT) 
 Time elapsed: 0.03 sec  <<< FAILURE!
java.lang.AssertionError: expected:<2> but was:<0>
        at org.junit.Assert.fail(Assert.java:93)
        at org.junit.Assert.failNotEquals(Assert.java:647)
        at org.junit.Assert.assertEquals(Assert.java:128)
        at org.junit.Assert.assertEquals(Assert.java:472)
        at org.junit.Assert.assertEquals(Assert.java:456)
        at 
org.apache.jackrabbit.oak.plugins.document.mongo.CacheInvalidationIT.testCacheInvalidationHierarchical(CacheInvalidationIT.java:131)