End test immediately when all number of blocks/bytes sent/received

[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): Lower throughtput numbers reported with v3.16 #1768

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

Currently, after to change to multi-thread, when limiting the test by number of bytes (-n) or number of blocks (-k), the test ends only when the current interval (-i) ends and not when all the data was sent/received. This is because the select() no longer monitor the steams data. Therefore the throughput statistics are not properly calculated. I.e. for 1 second interval, if sending was done in 2.5 seconds, the statistics will be as if sending was done in 3 seconds. This is much worse if the interval period is larger.

This suggested fix is using the self-pipe trick, based on this example. When -n or -k are set, a self-pipe is added to the Client's select. When the limit is reached, one byte is written to the pipe to wake-up the select() in the main thread.

[bmah888]

Thanks for the PR! Admittedlly this might take alittle time to study what this change does.