Add option to send different randomized blocks

[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): iperf3: use incompressible data for streaming test #1871

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

Add a second option to -l to specify number of different (randomized) blocks to send. The purpose is that not all sent blocks will be the same, and as the blocks contents is randomized by default, network compression will not have an effect.

Test, that stored the received data for 1KB blocks into a file, shows that when sending the same block contents, zip of the file achieves more than 99% compression. On the other hand, when the block contents repeats only every 100 blocks, zip achieves almost no compression.

The change includes also initializing a stream buffer only when it is a sending stream.

[bmah888]

Thanks for the PR, we'll take a look!