Run initial scan immediately (#123)

Rather than starting the shard scan loop and waiting for next scan tick before
fetching any data, do the first poll immediately, and then wait.

consumer.go

@@ -158,10 +158,6 @@ func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn ScanFunc) e
 	defer scanTicker.Stop()
 
 	for {
-		select {
-		case <-ctx.Done():
-			return nil
-		case <-scanTicker.C:
 		resp, err := c.client.GetRecords(&kinesis.GetRecordsInput{
 			Limit:         aws.Int64(c.maxRecords),
 			ShardIterator: shardIterator,
@@ -181,10 +177,7 @@ func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn ScanFunc) e
 			if err != nil {
 				return fmt.Errorf("get shard iterator error: %v", err)
 			}
-
-				continue
-			}
-
+		} else {
 			// loop over records, call callback func
 			for _, r := range resp.Records {
 				select {
@@ -214,6 +207,14 @@ func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn ScanFunc) e
 
 			shardIterator = resp.NextShardIterator
 		}
+
+		// Wait for next scan
+		select {
+		case <-ctx.Done():
+			return nil
+		case <-scanTicker.C:
+			continue
+		}
 	}
 }