Add numberOfRetries option

Add a numberOfRetries method to the DeadLetterEntry. This allows users a
 chance to evict the entry after a certain amount if they desire so.
Also, drop the GenericEventDeadLetter for now, as we're not using it at
  all. Lastly, fix the test entry in the DeadLetterEntryTest to comply
 with the compareTo change using the expiresAt field instead of the
 deadLettered field for comparison.

#2021

messaging/src/main/java/org/axonframework/messaging/deadletter/DeadLetterEntry.java

@@ -68,6 +68,13 @@ public interface DeadLetterEntry<T extends Message<?>> {
      */
     Instant expiresAt();
 
+    /**
+     * The number of retries this {@link DeadLetterEntry dead-letter} has gone through.
+     *
+     * @return The number of retries this {@link DeadLetterEntry dead-letter} has gone through.
+     */
+    int numberOfRetries();
+
     /**
      * Acknowledges this {@link DeadLetterEntry dead-letter} as successfully evaluated. This operation will remove the
      * entry from its queue.

messaging/src/main/java/org/axonframework/messaging/deadletter/InMemoryDeadLetterQueue.java

@@ -339,6 +339,7 @@ private class GenericDeadLetterMessage implements DeadLetterEntry<T> {
         private final T message;
         private final Throwable cause;
         private final Instant expiresAt;
+        private final int numberOfRetries;
         private final Instant deadLettered;
         private final Consumer<GenericDeadLetterMessage> releaseOperation;
 
@@ -350,6 +351,7 @@ private GenericDeadLetterMessage(DeadLetterEntry<T> entry,
                  entry.cause(),
                  entry.deadLettered(),
                  expiresAt,
+                 entry.numberOfRetries() + 1,
                  releaseOperation);
         }
 
@@ -359,11 +361,22 @@ private GenericDeadLetterMessage(QueueIdentifier queueIdentifier,
                                          Instant deadLettered,
                                          Instant expiresAt,
                                          Consumer<GenericDeadLetterMessage> releaseOperation) {
+            this(queueIdentifier, message, cause, deadLettered, expiresAt, 0, releaseOperation);
+        }
+
+        private GenericDeadLetterMessage(QueueIdentifier queueIdentifier,
+                                         T message,
+                                         Throwable cause,
+                                         Instant deadLettered,
+                                         Instant expiresAt,
+                                         int numberOfRetries,
+                                         Consumer<GenericDeadLetterMessage> releaseOperation) {
             this.queueIdentifier = queueIdentifier;
             this.message = message;
             this.cause = cause;
             this.deadLettered = deadLettered;
             this.expiresAt = expiresAt;
+            this.numberOfRetries = numberOfRetries;
             this.releaseOperation = releaseOperation;
         }
 
@@ -392,6 +405,11 @@ public Instant expiresAt() {
             return expiresAt;
         }
 
+        @Override
+        public int numberOfRetries() {
+            return numberOfRetries;
+        }
+
         @Override
         public void acknowledge() {
             releaseOperation.accept(this);
@@ -411,7 +429,7 @@ public boolean equals(Object o) {
                 return false;
             }
 
-            // Validation does not include the expiresAt and releaseOperation, to allow stable removal of entries
+            // Check does not include the expiresAt, numberOfRetries, and releaseOperation allowing easy entry removal.
             //noinspection unchecked
             GenericDeadLetterMessage that = (GenericDeadLetterMessage) o;
             return Objects.equals(queueIdentifier, that.queueIdentifier)
@@ -427,12 +445,13 @@ public int hashCode() {
 
         @Override
         public String toString() {
-            return "DeadLetterEntry{" +
+            return "GenericDeadLetterMessage{" +
                     "queueIdentifier=" + queueIdentifier +
                     ", message=" + message +
                     ", cause=" + cause +
-                    ", deadLettered=" + deadLettered +
                     ", expiresAt=" + expiresAt +
+                    ", numberOfRetries=" + numberOfRetries +
+                    ", deadLettered=" + deadLettered +
                     '}';
         }
     }

messaging/src/test/java/org/axonframework/messaging/deadletter/DeadLetterEntryTest.java

@@ -62,10 +62,10 @@ void testCompareReturnsZeroIfFirstAndSecondLetterOccurredAtTheSameTime() {
 
     private static class TestDeadLetterEntry implements DeadLetterEntry<Message<?>> {
 
-        private final Instant deadLettered;
+        private final Instant expiresAt;
 
-        private TestDeadLetterEntry(Instant deadLettered) {
-            this.deadLettered = deadLettered;
+        private TestDeadLetterEntry(Instant expiresAt) {
+            this.expiresAt = expiresAt;
         }
 
         @Override
@@ -85,12 +85,17 @@ public Throwable cause() {
 
         @Override
         public Instant expiresAt() {
-            return null;
+            return expiresAt;
+        }
+
+        @Override
+        public int numberOfRetries() {
+            return 0;
         }
 
         @Override
         public Instant deadLettered() {
-            return deadLettered;
+            return null;
         }
 
         @Override