many: add simple performance measure tool mostly for firstboot
This PR adds a very simplisit "measure" module to the tree that
can be used to take measurements of things that take time.
The API is minimal, the intended use (for now) is:
```
func foo() {
m := measure.New("things happening in foo")
defer m.Done()
...
}
```
The PR adds some measures into the firstboot code but we need to
experiment a bit too see where else we need to measure.
With that there is a new `snap debug measures` command that can
be used to get the most recent measurements.
We were using uint32 for pids in daemon and polkit, when they're
actually signed. This would be mostly transparent to snapd, but could
lead to spurious denials from polkit in some situations.