Before starting work on delete topic support, uploading a patch first to address some minor hiccups that touch a bunch of files:
1. Change PartitionOfflineException to PartitionUnavailableException because in the partition state machine we mark a partition offline when its leader is down, whereas the PartitionOfflineException is thrown when all the assigned replicas of the partition are down.
2. Change PartitionOfflineRate to UnavailablePartitionRate
3. Remove default leader selector from partition state machine's handleStateChange. We can specify null as default when we don't need to use a leader selector.
4. Include controller info in the client id of LeaderAndIsrRequest.
5. Rename controllerContext.allleaders to something more meaningful - partitionLeadershipInfo.
6. We don't need to put partition in OnlinePartition state in partition state machine initializeLeaderAndIsrForPartition, the state change occurs in handleStateChange.
7. Add todo in handleStateChanges
8. Left a comment above ReassignedPartitionLeaderSelector that reassigned replicas are already in the ISR (this is not true for other leader selectors), renamed the vals in the selector.