NC-1815 Rinkeby import can stall with too many fragments

Move "complete what we just got" semantic into the
DownloadHeaderSequenceTask.  Previously it was completing everything
in the chunk it was assigned.

ethereum/eth/src/main/java/tech/pegasys/pantheon/ethereum/eth/sync/tasks/DownloadHeaderSequenceTask.java

@@ -114,7 +114,7 @@ protected CompletableFuture<List<BlockHeader>> executePeerTask() {
     return task.whenComplete(
         (r, t) -> {
           // We're done if we've filled all requested headers
-          if (r != null && r.size() == segmentLength) {
+          if (lastFilledHeaderIndex == 0) {
             LOG.debug(
                 "Finished downloading headers from {} to {}.",
                 startingBlockNumber,
@@ -162,6 +162,7 @@ private CompletableFuture<List<BlockHeader>> processHeaders(
           final CompletableFuture<List<BlockHeader>> future = new CompletableFuture<>();
           BlockHeader child = null;
           boolean firstSkipped = false;
+          final int previousHeaderIndex = lastFilledHeaderIndex;
           for (final BlockHeader header : headersResult.getResult()) {
             final int headerIndex =
                 Ints.checkedCast(
@@ -191,7 +192,7 @@ private CompletableFuture<List<BlockHeader>> processHeaders(
             lastFilledHeaderIndex = headerIndex;
             child = header;
           }
-          future.complete(asList(headers).subList(lastFilledHeaderIndex, segmentLength));
+          future.complete(asList(headers).subList(lastFilledHeaderIndex, previousHeaderIndex));
           return future;
         });
   }