[Juggler] Drop callbacks, use Promises only

[bajtos]

As a part of the overall juggler refactor, we should drop all callback-based APIs (possibly with the exception of EventEmitters) in favor of Promise-based APIs that are compatible with async/await.

As part of this rework, we should drop support for Node.js versions older than 8.x and start using async/await for flow control.

See also #537 (comment)

[bajtos]

See also loopbackio/loopback-datasource-juggler#1659

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