Move AdvancedCacheLoader over to using Publisher instead of process

Currently our process method on AdvancedCacheLoader is very hard to write for proper thread usage, see [1]. There is no way to utilize the main thread to help with processing as well, except by always using a WithinThreadExecutor, which prevents any parallelism.

Instead we should have the loader expose a Producer which can be properly controlled and have backpressure built in (thus not wasting threads or resources like it does currently.

[1] https://github.com/infinispan/infinispan/blob/master/core/src/main/java/org/infinispan/persistence/util/PersistenceManagerCloseableSupplier.java#L110