types: make DataSource.stop compatible with LB4

[bajtos]

This is a follow-up for #1835.

Change the return value of DataSource.stop() from Promise<void> to void | PromiseLike<void> to avoid breaking existing LoopBack 4 applications, where DataSource subclasses are scaffolded with stop(): ValueOrPromise<void>.

Before the change, build of loopback-next was failing with the following errors (see loopbackio/loopback-next#5261)

https://travis-ci.com/github/strongloop/loopback-next/jobs/323793285

> loopback-next@0.1.0 build /home/travis/build/strongloop/loopback-next
> node packages/build/bin/compile-package -b

examples/multi-tenancy/src/datasources/db.datasource.ts:40:3 - error TS2416: Property 'stop' in type 'DbDataSource' is not assignable to the same property in base type 'DataSource'.
  Type '() => ValueOrPromise<void>' is not assignable to type '() => Promise<void>'.
    Type 'ValueOrPromise<void>' is not assignable to type 'Promise<void>'.
      Type 'void' is not assignable to type 'Promise<void>'.

40   stop(): ValueOrPromise<void> {
     ~~~~
(...)

I have manually verified that the new typings are fixing loopback-next build:

$ (cd packages/repository && npm link {path-to-juggler})
$ npm run build

Checklist

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• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• Commit messages are following our guidelines