[Minor] Start BlockPropagationManager immediately - don't wait for full sync (#1398)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

ethereum/eth/src/main/java/tech/pegasys/pantheon/ethereum/eth/sync/DefaultSynchronizer.java

@@ -105,7 +105,9 @@ public class DefaultSynchronizer<C> implements Synchronizer {
   @Override
   public void start() {
     if (running.compareAndSet(false, true)) {
+      LOG.info("Starting synchronizer.");
       syncState.addSyncStatusListener(this::syncStatusCallback);
+      blockPropagationManager.start();
       if (fastSyncDownloader.isPresent()) {
         fastSyncDownloader.get().start().whenComplete(this::handleFastSyncResult);
       } else {
@@ -118,8 +120,8 @@ public class DefaultSynchronizer<C> implements Synchronizer {
 
   @Override
   public void stop() {
-    LOG.info("Stopping synchronizer");
     if (running.compareAndSet(true, false)) {
+      LOG.info("Stopping synchronizer");
       fastSyncDownloader.ifPresent(FastSyncDownloader::stop);
       fullSyncDownloader.stop();
     }
@@ -148,8 +150,6 @@ public class DefaultSynchronizer<C> implements Synchronizer {
   }
 
   private void startFullSync() {
-    LOG.info("Starting synchronizer.");
-    blockPropagationManager.start();
     fullSyncDownloader.start();
   }