Improve controller failover latency

During controller failover, we do the following things -

1. Increment controller epoch
2. Initialize controller context
3. Initialize replica state machine
4. Initialize partition state machine

During step 2 above, we read the information of all topics and partitions, the replica assignments and leadership information. During step 3 and 4, we re-read some of this information from zookeeper. Since the zookeeper reads are proportional to the number of topics and the reads are serial, it is important to optimize this. The zookeeper reads in steps 3 and 4 are not required.