Bluebird library for promise management shouldn't be used anymore

[Freezystem]

As the bluebird github repo states, bluebird shouldn't be used anymore.
Moreover, the library hasn't got any release since december 2019.

Promises are available in nodeJS since v0.12.0 and the minimum version moleculer supports is nodeJS v8.x.x so this should be safe regarding backward compatibility issues.

We eventually could keep it as devDependency for tests purpose but resort to using native promise in adapters.

I'm willing to make the related PR if it's something that you think should be done.

[icebob]

Good idea, a PR would be good, thanks!

[esatterwhite]

I think in most cases you don't need to use promises directly anymore except in edge cases. An async function returns a promise inherently.

[Freezystem]

Here are the related PRs:

• Remove bluebird from moleculer-db-adapter-mongoose #386
• Remove bluebird from moleculer-db-adapter-mongo #387
• Remove bluebird from moleculer-db-adapter-sequelize #390
• Remove bluebird from moleculer-db-adapter-couchdb-nano #391
• Remove bluebird from moleculer-db #392
• Remove bluebird from moleculer-db-adapter-knex #393

To avoid changing the code too much I just removed bluebird but still use Promise.