NC-1815 Rinkeby import can stall with too many fragments (#224)

Quick fix - up the retry limit to 20, the observed chunking size at the point of failure.
Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

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

@@ -44,7 +44,7 @@ import org.apache.logging.log4j.Logger;
  */
 public class CompleteBlocksTask<C> extends AbstractRetryingPeerTask<List<Block>> {
   private static final Logger LOG = LogManager.getLogger();
-  private static final int DEFAULT_RETRIES = 3;
+  private static final int DEFAULT_RETRIES = 20;
 
   private final EthContext ethContext;
   private final ProtocolSchedule<C> protocolSchedule;

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

@@ -47,7 +47,7 @@ import org.apache.logging.log4j.Logger;
  */
 public class DownloadHeaderSequenceTask<C> extends AbstractRetryingPeerTask<List<BlockHeader>> {
   private static final Logger LOG = LogManager.getLogger();
-  private static final int DEFAULT_RETRIES = 3;
+  private static final int DEFAULT_RETRIES = 20;
 
   private final EthContext ethContext;
   private final ProtocolContext<C> protocolContext;