Reduce "Block creation processed cancelled" log message to debug so it doesn't print every time we import a block. (#294)

ethereum/blockcreation/src/main/java/tech/pegasys/pantheon/ethereum/blockcreation/BlockMiner.java

@@ -70,7 +70,7 @@ public class BlockMiner<C, M extends AbstractBlockCreator<C>> implements Runnabl
       try {
         blockMined = mineBlock();
       } catch (final CancellationException ex) {
-        LOG.info("Block creation process cancelled.");
+        LOG.debug("Block creation process cancelled.");
         break;
       } catch (final InterruptedException ex) {
         LOG.error("Block mining was interrupted.", ex);