Gap delay time between messages option

[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):

• None

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

Add an option --gap for setting random delay gap time between packets, instead of defining target bit rate. This is to allow creating test traffic profiles other then just fixed bit rate (and fixed delay between packets).

--gap receives two parameters n1[/n2]. The delay is a random number between n1 and n2. If only n1 is defined, then n2 is set to 5 times n1. For fixed delay (equivalent for setting bit rate), n1 and n2 are set for the same value. By default the gap value unit is for Mili-sec, but it can also be defined as Seconds or Micro-secs (Sec/Mili/Micro units are specified by s/m/u).

When --gap is set, burst default value 1 packet, to have the gap delay between each two packets. If bust option is set (using -b /<burst>) the the gap delay is between bursts, not between packets.

(This PR is a subset of the enhancements suggested by PR #1075, but with enhanced implementation - using the select timeout mechanism instead of sleep.)