Nested sampler approach needs review

**Sebb** ([Bug 51983](https://bz.apache.org/bugzilla//show_bug.cgi?id=51983&redirect=false)):
Some samplers support generation of multiple samples where there is a parent sample and various sub-samples. For example, HTTP redirects and HTTP embedded resource downloads.

In these cases, the parent sample should have some fields such as the the name and start-time taken from the initial sample, but other fields such as encoding, content-type etc. don't really make sense for the parent. However, at present the parent is created by cloning the initial saample.

Need to review which fields should be initialised, and which should not, and then add a new static factory method which performs the required init and use that instead of the current copy constructor.

OS: Windows XP