RocksDB purge should not utilise ArrayList to store multiple expired keys

Currently the RocksDB purge method stores multiple keys that expire at the same time in a ArrayList associated with the expiry time. This is currently marshallable because ArrayList is serializable and we utilise Jboss-marshalling, however ISPN-10354 means this is no longer possible in embedded mode when `infinispan-jboss-marshalling` is not present. Therefore, we should create a `ExpiryBucket` that can be marshalled via the persistence marshaller i.e. protobuf based.