REST server invokes potentially blocking operations on Netty threads

DefaultCacheManager.getCache(cacheName) is potentially blocking, and the REST server directly and indirectly invokes getCache(cacheName) on Netty threads.

BlockHound does not catch it in the test suite because caches are usually predefined and are already started by the time the test invokes any REST operations. But that's not always the case: when a cache is created dynamically, REST requests (like manager health requests from k8s liveness checks) may try to access the cache while it's starting and block.