Only compute "next" online account signature if mempow hard fork is active.

This minimizes the amount of differences in the first phase of the mempow rollout.

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

@@ -462,13 +462,15 @@ public class OnlineAccountsManager {
             return;
         }
 
-        // 'next' timestamp (prioritize this as it's the most important)
+        // 'next' timestamp (prioritize this as it's the most important, if mempow active)
         final long nextOnlineAccountsTimestamp = toOnlineAccountTimestamp(now) + getOnlineTimestampModulus();
+        if (nextOnlineAccountsTimestamp >= BlockChain.getInstance().getOnlineAccountsMemoryPoWTimestamp()) {
             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
                 return;
             }
+        }
 
         // 'current' timestamp
         computeOurAccountsForTimestamp(onlineAccountsTimestamp);