Fix iperf3 client hangs

[oagniqgnat]

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

• Issues fixed (if any):

  1. iperf3 client hangs after completing -n 50MB transfer, server reports 0.00 Bytes/sec continuously #2003
  2. Translate the cancellation type of the child threads from PTHREAD_CANCEL_ASYNCHRONOUS to PTHREAD_CANCEL_DEFERRED #1991

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

  1. @oagniqgnat Translate the cancellation type of the child threads from PTHREAD_CANCEL_ASYNCHRONOUS to PTHREAD_CANCEL_DEFERRED
  2. @davidBar-On Update iperf_pthread.h for Android support