Profiling - topics 'load-loader' and 'show' are not actually profiled

Summary

_profile.py lists some topics that we're able to profile (and produce logs and .cprofile binaries for).

However, two of the specified topics, 'show' and 'load-loader', have not had the relevant logic added to the codebase so that we can actually profile these two topics.

This simply requires calling Profile.profile_start() and Profile.profile_end() in the appropriate places, for example, for profiling the loading of the pipeline: https://gitlab.com/BuildStream/buildstream/blob/master/buildstream/_pipeline.py#L101

I've opened this issue to determine whether we should:

1. Keep these topics and profile them
2. Remove these topics