解决session在重启后，内存中不存在的问题

broker/src/main/java/io/moquette/persistence/MemorySessionStore.java

@@ -282,7 +282,7 @@ public class MemorySessionStore implements ISessionsStore {
 
 
     @Override
-    public ErrorCode createNewSession(String username, String clientID, boolean cleanSession) {
+    public ErrorCode createNewSession(String username, String clientID, boolean cleanSession, boolean createWhenNoExist) {
         LOG.debug("createNewSession for client <{}>", clientID);
 
         Session session = sessions.get(clientID);
@@ -296,6 +296,10 @@ public class MemorySessionStore implements ISessionsStore {
         session = databaseStore.getSession(username, clientID, clientSession);
 
         if (session == null) {
+            if (!createWhenNoExist) {
+                return ErrorCode.ERROR_CODE_NOT_EXIST;
+            }
+
             session = databaseStore.createSession(username, clientID, clientSession);
         }