Faster dev startup

Created by: eseliger

Please review commit by commit.

This should reduce time taken by ./dev/start.

• Removed apparently unnecessary second yarn run
• Fixed a nginx config error that caused 502s to be delivered longer than needed I realized that when I was working on the go frontend service, it was already up but the GQL explorer still received 502s. This is immediate now
• Parallelized root and LSIF yarn cc @efritz or was this intentional? 🤔
• Made tsc -b a background process as it's not needed for startup of the dev stack, just for code intelligence while developing. This shaves off another at least 10s, if a lot changed possibly even 20-30.