When using the `MessageChannelPartitionHandler`, the reply channel is an
instance variable. However, when the handle method is called, if a
reply channel is not injected, it will create a new one. If multiple
jobs recycle the same instance of the partition handler, you may end up
having messages crossed/lost. This PR removes the creation of the reply
channel for each call to the handle method and documents that this
comonent should be step scoped.
Resolves BATCH-2288
ad5348f32d