Race condition between mod_cluster jvm route auto-generation and DIST mode affinity logic can cause NPE

Fix to AS7-4560 has uncovered a race condition between mod_cluster's jvm route auto-generation and the DIST mode affinity logic in the web session replication code.

08:19:20,728 ERROR [org.apache.catalina.connector.CoyoteAdapter] (ajp--172.17.39.254-8159-10) An exception or error occurred in the container during the request processing: java.lang.NullPointerException
at org.jboss.as.clustering.web.infinispan.DistributedCacheManager.locate(DistributedCacheManager.java:420)
at org.jboss.as.web.session.DistributableSessionManager.locate(DistributableSessionManager.java:815) [jboss-as-web-7.1.2.Final-SNAPSHOT.jar:7.1.2.Final-SNAPSHOT]
at org.jboss.as.web.session.JvmRouteValve.handleJvmRoute(JvmRouteValve.java:147) [jboss-as-web-7.1.2.Final-SNAPSHOT.jar:7.1.2.Final-SNAPSHOT]
at org.jboss.as.web.session.JvmRouteValve.checkJvmRoute(JvmRouteValve.java:105) [jboss-as-web-7.1.2.Final-SNAPSHOT.jar:7.1.2.Final-SNAPSHOT]
at org.jboss.as.web.session.JvmRouteValve.invoke(JvmRouteValve.java:85) [jboss-as-web-7.1.2.Final-SNAPSHOT.jar:7.1.2.Final-SNAPSHOT]
at org.jboss.as.web.session.LockingValve.invoke(LockingValve.java:56) [jboss-as-web-7.1.2.Final-SNAPSHOT.jar:7.1.2.Final-SNAPSHOT]
at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153) [jboss-as-web-7.1.2.Final-SNAPSHOT.jar:7.1.2.Final-SNAPSHOT]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155) [jbossweb-7.0.15.Final.jar:]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [jbossweb-7.0.15.Final.jar:]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [jbossweb-7.0.15.Final.jar:]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368) [jbossweb-7.0.15.Final.jar:]
at org.apache.coyote.ajp.AjpAprProcessor.process(AjpAprProcessor.java:490) [jbossweb-7.0.15.Final.jar:]
at org.apache.coyote.ajp.AjpAprProtocol$AjpConnectionHandler.process(AjpAprProtocol.java:480) [jbossweb-7.0.15.Final.jar:]
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:2039) [jbossweb-7.0.15.Final.jar:]
at java.lang.Thread.run(Thread.java:662) [rt.jar:1.6.0_31]