* fix: weavelet cpu consumption

after to run the hello example with `go run .` the cpu consumption was
at 100%, this change adds a blocking feature to `getcomponentstostart`
in `singleprocess.go`, this prevents the excessive cpu consumption

```
(pprof) top
showing nodes accounting for 29.98s, 100% of 29.99s total
dropped 7 nodes (cum <= 0.15s)
      flat  flat%   sum%        cum   cum%
    15.66s 52.22% 52.22%     29.98s   100%  github.com/serviceweaver/weaver.(*weavelet).watchcomponentstostart
     9.71s 32.38% 84.59%     11.14s 37.15%  github.com/serviceweaver/weaver/runtime/retry.(*retry).continue
     3.18s 10.60% 95.20%      3.18s 10.60%  github.com/serviceweaver/weaver.(*singleprocessenv).getcomponentstostart
     1.43s  4.77%   100%      1.43s  4.77%  context.(*emptyctx).err
         0     0%   100%     29.98s   100%  github.com/serviceweaver/weaver.startwork.func1
```

* Update singleprocess.go

Co-authored-by: Daniel Nephin <dnephin@gmail.com>

* Update singleprocess.go

Co-authored-by: Daniel Nephin <dnephin@gmail.com>

---------

Co-authored-by: Srdjan Petrovic <spetrovic@google.com>
Co-authored-by: Daniel Nephin <dnephin@gmail.com>
Co-authored-by: Robert Grandl <rgrandl@users.noreply.github.com>