Correct UDP link capability in JSON and CPU Utlization for Server Text output

[davidBar-On]

• Version of iperf3 (or development branch, such as master or 3.1-STABLE) to which this pull request applies: master

• Issues fixed (if any):

• #754

• #1552

• Brief description of code changes (suitable for use as a commit message):

Suggested fix for issue #754 - correct UDP JSON output. The main issue was on the server's side since the call to reporter_callback() was done prior to results exchange with the client. In addition, some enhancements were done to the reported received and sent bytes count.

Update - 26Jul2023: Added a suggested fix to #1552 since it also requires the same move of the call to reporter_callback(). Since this is a minor and related change, I believe that it is o.k. to handle both fixes together.