Fix ConcurrentModificationException in getCachedSigValidTransactions()

src/main/java/org/qortal/controller/TransactionImporter.java

@@ -99,11 +99,13 @@ public class TransactionImporter extends Thread {
      * @return a list of TransactionData objects, with valid signatures.
      */
     private List<TransactionData> getCachedSigValidTransactions() {
+        synchronized (this.incomingTransactions) {
             return this.incomingTransactions.entrySet().stream()
                     .filter(t -> Boolean.TRUE.equals(t.getValue()))
                     .map(Map.Entry::getKey)
                     .collect(Collectors.toList());
         }
+    }
 
     /**
      * Validate the signatures of any transactions pending import, then update their