!per #3707 Channel enhancements

- Persistent channel
- ConfirmablePersistent message type delivered by channel
- Sender resolution performance improvements
   * unstash() instead of unstashAll()

These enhancements required the following changes

- Unified implementation of processor stash and user stash
- Persistence message plugin API separated from implementation
- Physical deletion of messages

akka-docs/rst/java/code/docs/persistence/PersistencePluginDocTest.java

@@ -36,22 +36,22 @@ public class PersistencePluginDocTest {
 
     class MyAsyncJournal extends AsyncWriteJournal {
         @Override
-        public Future<Long> doReplayAsync(String processorId, long fromSequenceNr, long toSequenceNr, Procedure<PersistentImpl> replayCallback) {
+        public Future<Long> doReplayAsync(String processorId, long fromSequenceNr, long toSequenceNr, Procedure<PersistentRepr> replayCallback) {
             return null;
         }
 
         @Override
-        public Future<Void> doWriteAsync(PersistentImpl persistent) {
+        public Future<Void> doWriteAsync(PersistentRepr persistent) {
             return null;
         }
 
         @Override
-        public Future<Void> doWriteBatchAsync(Iterable<PersistentImpl> persistentBatch) {
+        public Future<Void> doWriteBatchAsync(Iterable<PersistentRepr> persistentBatch) {
             return null;
         }
 
         @Override
-        public Future<Void> doDeleteAsync(PersistentImpl persistent) {
+        public Future<Void> doDeleteAsync(String processorId, long sequenceNr, boolean physical) {
             return null;
         }