iperf3: use incompressible data for streaming test

Various access technologies, as well as transparent WAN accellerators, TCP proxies, VPNs and Encryption protocols willl try to compress (gz) the data stream and/or the packets. This is particularly notable by sending zeroed data payload, or when the data buffer which is used to send, is relatively small (few kB) - this can lead to results of throuhgput which are not attainable with normally compressible data, or incompressible data (e.g. media streaming data).

iperf3 should optionally have the capability to allocate a larger chunk of main memory, and fill this with random data or pre-compressed data (alternatively, consume a file / stdin with user-provided data to send, which is sent repeatedly when the data runs out before the test is over).