[BUG] admin_addPeer isn't periodically checking maintained connections (#871)

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

ethereum/p2p/src/main/java/tech/pegasys/pantheon/ethereum/p2p/NetworkRunner.java

@@ -90,7 +90,8 @@ public class NetworkRunner implements AutoCloseable {
       LOG.info("Starting Network.");
       setupHandlers();
       networkExecutor.submit(network);
-      networkCheckExecutor.schedule(network::checkMaintainedConnectionPeers, 60, TimeUnit.SECONDS);
+      networkCheckExecutor.scheduleWithFixedDelay(
+          network::checkMaintainedConnectionPeers, 60, 60, TimeUnit.SECONDS);
     } else {
       LOG.error("Attempted to start already running network.");
     }