Make logic in PersistBlockTask more explicit to fix a LGTM warning (#92)

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

ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/tasks/PersistBlockTask.java

@@ -27,6 +27,7 @@ import org.hyperledger.besu.ethereum.mainnet.ProtocolSpec;
 import org.hyperledger.besu.plugin.services.MetricsSystem;
 
 import java.util.ArrayList;
+import java.util.Iterator;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
 import java.util.function.Supplier;
@@ -67,22 +68,20 @@ public class PersistBlockTask<C> extends AbstractEthTask<Block> {
       final List<Block> blocks,
       final HeaderValidationMode headerValidationMode,
       final MetricsSystem metricsSystem) {
-    checkArgument(blocks.size() > 0);
+    checkArgument(!blocks.isEmpty(), "No blocks to import provided");
     return () -> {
       final List<Block> successfulImports = new ArrayList<>();
-      CompletableFuture<Block> future = null;
-      for (final Block block : blocks) {
-        if (future == null) {
-          future =
-              importBlockAndAddToList(
-                  protocolSchedule,
-                  protocolContext,
-                  block,
-                  successfulImports,
-                  headerValidationMode,
-                  metricsSystem);
-          continue;
-        }
+      final Iterator<Block> blockIterator = blocks.iterator();
+      CompletableFuture<Block> future =
+          importBlockAndAddToList(
+              protocolSchedule,
+              protocolContext,
+              blockIterator.next(),
+              successfulImports,
+              headerValidationMode,
+              metricsSystem);
+      while (blockIterator.hasNext()) {
+        final Block block = blockIterator.next();
         future =
             future.thenCompose(
                 b ->
@@ -122,34 +121,34 @@ public class PersistBlockTask<C> extends AbstractEthTask<Block> {
       final List<Block> blocks,
       final HeaderValidationMode headerValidationMode,
       final MetricsSystem metricsSystem) {
-    checkArgument(blocks.size() > 0);
+    checkArgument(!blocks.isEmpty(), "No blocks to import provided");
     return () -> {
       final CompletableFuture<List<Block>> finalResult = new CompletableFuture<>();
       final List<Block> successfulImports = new ArrayList<>();
-      CompletableFuture<Block> future = null;
-      for (final Block block : blocks) {
-        if (future == null) {
-          future =
-              PersistBlockTask.create(
-                      protocolSchedule, protocolContext, block, headerValidationMode, metricsSystem)
-                  .run();
-          continue;
-        }
+      final Iterator<PersistBlockTask<C>> tasks =
+          blocks.stream()
+              .map(
+                  block ->
+                      PersistBlockTask.create(
+                          protocolSchedule,
+                          protocolContext,
+                          block,
+                          headerValidationMode,
+                          metricsSystem))
+              .iterator();
+
+      CompletableFuture<Block> future = tasks.next().run();
+      while (tasks.hasNext()) {
+        final PersistBlockTask<C> task = tasks.next();
         future =
             future
                 .handle((r, t) -> r)
                 .thenCompose(
-                    (r) -> {
+                    r -> {
                       if (r != null) {
                         successfulImports.add(r);
                       }
-                      return PersistBlockTask.create(
-                              protocolSchedule,
-                              protocolContext,
-                              block,
-                              headerValidationMode,
-                              metricsSystem)
-                          .run();
+                      return task.run();
                     });
       }
       future.whenComplete(