Fixed #1963 - DB023 - Crazy amount of dbListenerTokens - makes the DB run slow (#1694)

- LiveQuery with `Query.setParameters(...)` calls `LiveQuery.start()`. In `start()` method, registering Database Listener every time. If already Database Listener is registered, it is not necessary to new one.

Author: Hideki Itakura

shared/src/main/java/com/couchbase/lite/LiveQuery.java

@@ -119,7 +119,10 @@ void start() {
             observing = true;
             releaseResultSet();
             query.getDatabase().getActiveLiveQueries().add(this);
-            dbListenerToken = query.getDatabase().addChangeListener(this);
+            // NOTE: start() method could be called during LiveQuery is running.
+            // Ex) Query.setParameters() with LiveQuery.
+            if(dbListenerToken == null)
+                dbListenerToken = query.getDatabase().addChangeListener(this);
             update(0);
         }
     }
@@ -131,8 +134,10 @@ void stop(boolean removeFromList) {
         synchronized (lock) {
             observing = false;
             willUpdate = false; // cancels the delayed update started by -databaseChanged
-            if (query != null && query.getDatabase() != null)
+            if (query != null && query.getDatabase() != null && dbListenerToken != null) {
                 query.getDatabase().removeChangeListener(dbListenerToken);
+                dbListenerToken = null;
+            }
             if (removeFromList && query != null && query.getDatabase() != null)
                 query.getDatabase().getActiveLiveQueries().remove(this);
             releaseResultSet();