Generalize apollo-server graceful shutdown to all integrations

[glasser]

Previously, the batteries-included apollo-server package had a special
override of stop() which drains the HTTP server before letting the
actual Apollo Server stop() machinery begin. This meant that
apollo-server followed this nice shutdown lifecycle:

• Stop listening for new connections
• Close all idle connections and start closing connections as they go
  idle
• Wait a grace period for all connections to close and force-close any
  remaining ones
• Transition ApolloServer to the stopping state, where no operations
  will run
• Run stop hooks (eg send final usage report)

This was great... but only apollo-server worked this way, because only
apollo-server has full knowledge and control over its HTTP server.

This PR adds a server draining step to the ApolloServer lifecycle and
plugin interface, and provides a built-in plugin which drains a Node
http.Server using the logic of the first three steps above.
apollo-server's behavior is now just to automatically install the
plugin.

Specifically:

• Add a new 'phase' called draining that fits between started and
  stopping. Like started, operations can still execute during
  draining. Like stopping, any concurrent call to stop() will just
  block until the first stop() call finishes rather than starting a
  second shutdown process.
• Add a new drainServer plugin hook (on the object returned by
  serverWillStart). Invoke all drainServer hooks in parallel during
  the draining phase.
• Make calling stop() when start() has not yet completed
  successfully into an error. That behavior was previously undefined.
  Note that as of apollo-server-core: register signal handlers later and not on serverless #5639, the automatic stop() call from signal
  handlers can't happen before start() succeeds.
• Add ApolloServerPluginDrainHttpServer to apollo-server-core.
  This plugin implements drainServer using the Stopper class
  that was previously in the apollo-server package. The default
  grace period is 10 seconds.
• Clean up integration tests to just use stop() with the plugin
  instead of separately stopping the HTTP server. Note that for Fastify
  specifically we also call app.close although there is some weirdness
  here around both app.close and our Stopper closing the same server.
  A comment describes the weirdness; perhaps Fastify experts can improve
  this later.
• The Hapi web framework has built in logic that is similar to our
  Stopper, so apollo-server-hapi exports
  ApolloServerPluginStopHapiServer which should be used instead of the
  other plugin with Hapi.
• Remove some examples from READMEs and point to examples in the docs
  instead. Keeping both up to date is extra work.
• Fix some test issues (eg, have FakeTimers only mock out Date.now
  instead of setImmediate, drop an erroneous const which made an app
  not get cleaned up, etc).

Fixes #5074.

[IvanGoncharov]

@glasser Code level changes look good and the idea to drain the server as a separate phase also makes a lot of sense.

[glasser]

I'd like to move forward with releasing this, but it would be great to have a second round of post-merge eyes on this from @IvanGoncharov. And @StephenBarlow , you may have thoughts on the docs!