Improve support for tracing REST requests

[bobbingwide]

With the new block editor using REST requests to get the data to display in dynamic blocks and document blocks we need an improved capability of tracing REST requests.
Part of the solution is to support tracing of REST requests to a different set of files
We also need more information in the trace log.
The data to be traced is in the global $HTTP_RAW_POST_DATA.
This is not normally available until the REST API has done its bit.

Hooking into 'rest_pre_echo_response' allows us to see both the incoming request and the response.

[bobbingwide]

Also update the Tested up to section of the readme.txt file.