iperf3: updated to 3.7

3.7:
iperf 3.6 adds the --bidir flag for bidirectional tests, includes some minor enhancements, and fixes a number of bugs. More details can be found in the release notes.
Note: Documentation for the --bidir flag was inadvertently omitted from the manual page. This will be fixed in a future release.

3.6:
iperf 3.6 adds the --extra-data and --repeating-payload options and fixes some minor bugs.

3.5:
iperf 3.5 fixes a bug that could over-count data transfers (and hence measured bitrate).

3.4:
iperf 3.4 fixes a number of minor bugs and adds a few enhancements.

3.3:
New minor release of iperf 3.3, fixing a number of minor bugs.

3.2:
New minor release of iperf 3.2, with new features, bugfixes, and enhancements.

(adam)