Change wrk->stats.bla to wrk->stats->bla since it trips me up every
single time, and it is inconsistent with how we do it elsewhere.
Lend the worker a pointer to req/bo's vsl to allow us to simplify some
function arguments.
Trying to clean up various function arguments by moving towards
struct worker for "usual stuff" like vsl, stats etc.
Don't mix const and non-const pointers. This should make jenkins happy again.
More refactoring:  HSH_* functions take struct worker as first argument
Move Obj* API towards taking a struct worker as first argument.
More refactoring to make struct worker the default context for Obj*()
Final refactoring of struct worker as context for Obj*()