Fetch shard IDs from Kinesis if DynamoDB cache is empty (#22)

* Skip shard ID cache on first run

On startup, if the metadata table is being created for the
first time, the loadShardIDsFromDynamo() call in refreshShards()
will always return an empty shard list.

This causes the consumer to wait until the next leader
election to start receiving events, because that's the only
time loadShardIDsFromKinesis() is called. By default, this happens
once a minute, which means the consumer basically sleeps for 1
minute on first run.

This commit fixes that by adding a first run flag and skipping the
cache if it's true.

Fixes #17.

* Remove first run check

kinsumer.go

@@ -157,6 +157,18 @@ func (k *Kinsumer) refreshShards() (bool, error) {
 	}
 
 	shardIDs, err = loadShardIDsFromDynamo(k.dynamodb, k.metadataTableName)
+
+	if err != nil {
+		return false, err
+	}
+
+	if len(shardIDs) == 0 {
+		shardIDs, err = loadShardIDsFromKinesis(k.kinesis, k.streamName)
+		if err == nil {
+			err = k.setCachedShardIDs(shardIDs)
+		}
+	}
+
 	if err != nil {
 		return false, err
 	}
@@ -250,6 +262,7 @@ func (k *Kinsumer) dynamoCreateTableIfNotExists(name, distKey string) error {
 	if k.dynamoTableExists(name) {
 		return nil
 	}
+
 	_, err := k.dynamodb.CreateTable(&dynamodb.CreateTableInput{
 		AttributeDefinitions: []*dynamodb.AttributeDefinition{{
 			AttributeName: aws.String(distKey),