This reverts commit 771705f, which
reverted 2c48b87 because it was
breaking some test cases.

Coming back to this as a starting point to try to fix the tests and add
new tests for the graceful shutdown feature.

- remove immediate process.exit from `next start`
- have handleSessionStop wait for the child to exit
- add a test that verifies long-running api routes are not killed
- send proper signal (e.g. SIGTERM or SIGINT) to child process from dev
  server
- wait for app to shut down in tests, with a short default timeout

- when a SIGINT or SIGTERM is sent to next dev, shutdown the server immediately
- create new test app for graceful-shutdown features
- add some more tests

- shouldn't accept requests while server is shutting down and there's
  no activity
- refactor some tests to use process.kill since it's faster and we
  aren't looking to kill the whole tree. makes killApp simpler too

- moved some js files to ts and fixed types
- fixed standalone server graceful-shutdown
- added new tests for production standalone mode

rather than having an integration test with dev and production modes,
and then a separate test for standalone-mode that copies all the same
tests, we can just follow the same pattern we had in integration but
include the standalone-mode tests in the same file.

the tsconfig.json file was causing typescript issues when looking for e2e-utils and next-test-utils

keeping code refactor to use "once", but adding that extra check broke a lot of tests