Use 'md' argument to plans instead of mutating RE.md inside plans.

There are 140 occurrences of `RE.md` inside the profile, mostly plans modifying it. It would be better to do

```
def my_plan(...):
    yield from count(..., md={...})
```

rather than

```py
def my_plan(...):
    RE.md.update({...})
    yield from count(...)
```