Allow nonce to be computed for "next" timestamp if mempow is enabled in settings.

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

@@ -464,7 +464,7 @@ public class OnlineAccountsManager {
 
         // 'next' timestamp (prioritize this as it's the most important, if mempow active)
         final long nextOnlineAccountsTimestamp = toOnlineAccountTimestamp(now) + getOnlineTimestampModulus();
-        if (nextOnlineAccountsTimestamp >= BlockChain.getInstance().getOnlineAccountsMemoryPoWTimestamp()) {
+        if (isMemoryPoWActive(now)) {
             boolean success = computeOurAccountsForTimestamp(nextOnlineAccountsTimestamp);
             if (!success) {
                 // We didn't compute the required nonce value(s), and so can't proceed until they have been retried