Move RPC response serialization into Server.doResponse

In HDFS-13399 we addressed a race condition in AlignmentContext processing where the RPC response would assign a transactionId independently of the transactions own processing, resulting in a stateId response that was lower than expected. However this caused us to serialize the RpcResponse twice in order to address the header field change.

See here:
https://issues.apache.org/jira/browse/HDFS-13399?focusedCommentId=16464279&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-16464279

And here:
https://issues.apache.org/jira/browse/HDFS-13399?focusedCommentId=16498660&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-16498660

At the end it was agreed upon to move the logic of Server.setupResponse into Server.doResponse directly.