Replace Bluebird with native promises

[kibertoad]

As Matteo Collina once said, "The first rule of Node.js performance optimization is to remove bluebird" (https://twitter.com/matteocollina/status/1351949461371686914)

Considering that support for older Node versions was dropped, it should be fairly straightforward to adjust current implementation to fully utilize native promises instead.

If there is approval for this change, I can provide a PR implementing it.

[jimmywarting]

👍
Drop support for older node version, safe-buffer can be replaced with buffer from node

[kibertoad]

PR for this was already up: #624

[carlhoerberg]

https://github.com/cloudamqp/amqp-client.js/ is a newer AMQP library that uses native promises (along with other improvements)

[cressie176]

amqplib@0.10.0 uses native promises