Upstream receiver end intervals reporting wrong seconds value

# Context

* Version of iperf3:
3.6

* Hardware:
MacBookPro

* Operating system (and distribution, if any):
macOS

# Bug Report

* Expected Behavior

The end streams receiver seconds should be receiver end time - receiver start time.

* Actual Behavior

It looks like the receiver seconds is reporting the sender's seconds. This bug does not look like it impacts results.

* Steps to Reproduce

Run an upstream test and look at the end stream JSON.

```
"end": {
"streams": [{
"sender": {
"socket": 8,
"start": 0,
"end": 8.00232195854187,
"seconds": 8.00232195854187,
"bytes": 17896104,
"bits_per_second": 17890911.255723491
},
"receiver": {
"socket": 8,
"start": 0,
"end": 8.113577127456665, // <-- this should be the seconds for receiver
"seconds": 8.00232195854187, // <-- incorrect, but also same as sender's
"bytes": 17413648,
"bits_per_second": 17169884.726746753
}
}, {
"sender": {
"socket": 18,
"start": 0,
"end": 8.00232195854187,
"seconds": 8.00232195854187,
"bytes": 24251592,
"bits_per_second": 24244555.143511336
},
"receiver": {
"socket": 18,
"start": 0,
"end": 8.113577127456665, // <-- this should be the seconds for receiver
"seconds": 8.00232195854187, // <-- incorrect, but also same as sender's
"bytes": 23638600,
"bits_per_second": 23307697.336116809
}
}],
"sum_sent": {
"start": 0,
"end": 8.00232195854187,
"seconds": 8.00232195854187,
"bytes": 42147696,
"bits_per_second": 42135466.399234824
},
"sum_received": {
"start": 0,
"end": 8.113577127456665,
"seconds": 8.113577127456665,
"bytes": 41052248,
"bits_per_second": 40477582.062863566
},
"cpu_utilization_percent": {
"host_total": 6.3741933515607068,
"host_user": 1.589532044632852,
"host_system": 4.7846492005427539,
"remote_total": 5.0273987385438152,
"remote_user": 0.65463746550824486,
"remote_system": 4.5122774338426481
},
"receiver_tcp_congestion": "cubic"
}
```