associate request to thread

[nuno-silva]

The instrumentation code needs to know to what request a thread belongs to.
This will allow us to know that, for instance, request /r.html?f=test05.txt&sc=640&sr=360&wc=640&wr=360&coff=0&roff=0 needed 92069358 method calls, 29574356 allocations, etc.
A simple way to do this is for the server to store a <request, threadID> list which the instrumented code can then access before writing the metrics.

part of #10