add a getTopicMetaData method in broker and expose it to producer

We need a way to propagate the leader and the partition information to the producer so that it can do load balancing and semantic partitioning. One way to do that is to have the producer get the information from ZK directly. This means that the producer needs to maintain a ZK session and has to subscribe to watchers, which can be complicated. An alternative approach is to have the following api on the broker.

TopicMetaData getTopicMetaData(String: topic)

TopicMetaData {
Array[PartitionMetaData]: partitionsMetaData
}

PartitionMetaData {
Int: partitionId
String: leaderHostname
Int: leaderPort
}

Using this api, the producer can get the metadata about a topic during initial startup or leadership change of a partition.