AS7-6639 synchronize getCache() calls to workaround ISPN-2796

Fixes intermittent failures such as seen here:

http://lightning.mw.lab.eng.bos.redhat.com/jenkins/job/as7-param-pull/6228/testReport/junit/org.jboss.as.test.integration.jpa.secondlevelcache/JPA2LCTestCase/org_jboss_as_test_integration_jpa_secondlevelcache_JPA2LCTestCase/

until next Infinispan release is available.