Set context classloader for worker threads explicitly to prevent leaks

new Thread(...) creates a thread using the same context classloader as the current thread.
When application code invokes an operation that ends up creating an internal Infinispan thread, the internal thread ends up with the application thread's context classloader.

This may cause leaks in an environment where the application's classloader can be unloaded before the DefaultCacheManager is stopped (e.g. in an application server where the cache manager is created outside the deployment, or with Spring Boot devtools automatic restart).