apollo-server-lambda: support Node 14 runtime (#5004)

Lambda has two ways of defining a handler: as an async Promise-returning function, and as a callback-invoking function. https://docs.aws.amazon.com/lambda/latest/dg/nodejs-handler.html The async variety was introduced with Lambda's Node 8 runtime. Apparently the callback-invoking variety was removed with their Node 14 runtime (their docs don't mention this anywhere but our users have reported this: #1989 (comment)). While AWS doesn't directly support pre-Node-8 runtimes any more, it's possible some users are using a Custom Runtime that still requires the Node 6 version, and Apollo Server still technically supports Node 6. So we would like to support both an async and a callback-invoking handler.

To implement this, I rewrote the handler to be an async function and used a little maybeCallbackify function to make it work as either type of handler. (Apollo Server 3 will drop Node 6 support, at which point we should just make this package always return an async handler. Tracking in #5018.) This simplified a lot of the logic and I was able to replace various Promise and callback-y stuff with direct code. To make this easier, I inlined the graphqlLambda function (which was the main API in Apollo Server v1 but is not exported in v2). This makes it easier to avoid bugs like #3999.

(We aren't positive if there is actually a Node 14 compatibility issue or not; see #1989 (comment). It's still a helpful refactor though.)

Original version by @adikari.
Fixes #1989

Co-authored-by: David Glasser <glasser@davidglasser.net>

CHANGELOG.md

@@ -12,6 +12,7 @@ The version headers in this history reflect the versions of Apollo Server itself
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  With few exceptions, the format of the entry should follow convention (i.e., prefix with package name, use markdown `backtick formatting` for package names and code, suffix with a link to the change-set à la `[PR #YYY](https://link/pull/YYY)`, etc.).  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
 - `apollo-server-core`: The `SIGINT` and `SIGTERM` signal handlers installed by default (when not disabled by `stopOnTerminationSignals: false`) now stay active (preventing process termination) while the server shuts down, instead of letting a second signal terminate the process. The handlers still re-signal the process after `this.stop()` concludes. Also, if `this.stop()` throws, the signal handlers will now log and exit 1 instead of throwing an uncaught exception. [Issue #4931](https://github.com/apollographql/apollo-server/issues/4931)
+- `apollo-server-lambda`: (**UPDATE THIS MESSAGE BEFORE RELEASE**; we are not sure if this actually helps nodejs14 compatibility or if it's just a nice refactor.) Support the nodejs14 runtime by changing the handler to be an [async handler](https://docs.aws.amazon.com/lambda/latest/dg/nodejs-handler.html#nodejs-handler-async). (For backwards compatibility, if the handler receives a callback, it still acts like a non-async handler.) [Issue #1989](https://github.com/apollographql/apollo-server/issues/1989) [PR #5004](https://github.com/apollographql/apollo-server/pull/5004)
 
 ## v2.21.1