server_timer_proc - better handle server test time expiration

[davidBar-On]

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

• Issues fixed (if any): none

• Brief description of code changes (suitable for use as a commit message): Enhancements to server_timer_proc() for better handling of server test time expiration

[bmah888]

Thanks for the PR! Will review.