add trace profiling

[nkubala]

This will help us profile where the most optimizations can be made in container-diff.

To use, run container-diff with CONTAINER_DIFF_ENABLE_PROFILING=1:

➜  container-diff git:(profile) ✗ CONTAINER_DIFF_ENABLE_PROFILING=1 ./out/container-diff diff gcr.io/google-appengine/python:latest daemon://python_upgrade:latest --types=pip --alsologtostderr -v 10
2017/10/06 12:25:34 profile: trace enabled, /var/folders/sd/6b31fmr96q93wr2y82lqpb0h00cjy3/T/profile223722564/trace.out
I1006 12:25:34.622636    2829 diff.go:81] Starting diff on images gcr.io/google-appengine/python:latest and daemon://python_upgrade:latest, using differs: [pip]
...
...
I1006 12:26:56.573392    2829 image_prep_utils.go:164] Removing image filesystem directory /var/folders/sd/6b31fmr96q93wr2y82lqpb0h00cjy3/T/python_upgradelatest804814547 from system
I1006 12:26:58.455451    2829 image_prep_utils.go:164] Removing image filesystem directory /var/folders/sd/6b31fmr96q93wr2y82lqpb0h00cjy3/T/gcr.iogoogle-appenginepythonlatest496923158 from system
2017/10/06 12:27:00 profile: trace disabled, /var/folders/sd/6b31fmr96q93wr2y82lqpb0h00cjy3/T/profile223722564/trace.out

Then, run go tool trace <path_to_trace>/trace.out.

[r2d4]

Can you put this in main.go instead?

[r2d4]

.