replicas may not have consistent data after becoming follower

This is an issued introduced in KAFKA-1001. The issue is that in ReplicaManager.makeFollowers(), we truncate the log before marking the replica as the follower. New messages from the producer can still be added to the log after the log is truncated, but before the replica is marked as the follower. Those newly produced messages can actually be committed, which implies those truncated messages are also committed. However, the new leader is not guaranteed to have those truncated messages.