Infrastructure cleanup

[martijnwalraven]

This PR cleans up the build and testing infrastructure, similar to apollographql/apollo-server#1521.

• It updates Lerna to 3.x and converts links between packages to use file: specifiers, as recommended by the Lerna author. This requires moving all devDependencies to the top-level package. The big benefit of this is that hoisting and linking no longer relies on lerna bootstrap, but uses native npm mechanisms. Among other things, this gives us a consistent package-lock.json for the entire monorepo.
• As a consequence, in CI we can now use the much faster npm ci(which depends on package-lock.json).
• We also switch from Travis to Circle CI, using the package-lock.json checksum to cache the npm cache dir.
• Instead of invoking a separate Jest runner per package with lerna run test, we now use a single top-level Jest runner. This makes tests run a lot faster, and works really well in combination with file watching.
• To fix Jest in CI, we run with --maxWorkers=2. It turns out Jest by default uses the number of cores, which is way too much when running in a container, and leads to bad performance and failing tests.
• I've also cleaned up the package scripts and various other things I ran into to give us a more consistent environment.

[abernix]

This is amazing!

Due to the massiveness of the package-lock.json changes, I've merely skimmed over those changes, but I'm confident that the scope of those changes were neceesary due to various npm version updates (which change the format of the file) and the overall shifting of modules.

LGTM!