Sherlock, a new GitLab specific profiling tool

This is still very much a work in progress. No documentation and/or tests have been added yet, the UI is super rough, and the name might also still change.

Those wanting to try this out can checkout the branch and then start Rails by running:

ENABLE_SHERLOCK=1 bundle exec rails s

Recorded transactions can be found at /sherlock/transactions, Sherlock's own routes are not tracked.