x/build/{cmd/coordinator,app/appengine}: combine codebases

[dmitshur]

We've discussed and agreed wanting to merge app/appengine into cmd/coordinator. The motivation is to have a single codebase, share more code, fewer separate deployments, and tests with more inclusive coverage.

I didn't find an existing issue, so making this one to track the task.

/cc @bradfitz @andybons @toothrot

[bradfitz]

I regret that I have but four emojis to give to this bug.

[gopherbot]

Change https://golang.org/cl/199800 mentions this issue: app/appengine: don't exclude release-branch-only builders

[gopherbot]

Change https://golang.org/cl/199878 mentions this issue: app/appengine: hide release-branch-only builders when viewing master branch

[gopherbot]

Change https://golang.org/cl/208319 mentions this issue: app/appengine: remove ancient performance/benchmarking code

[gopherbot]

Change https://golang.org/cl/208320 mentions this issue: app/appengine, cmd/genbuilderkey: move key gen from App Engine to new tool