Add correct Types for PromisePoolCluster

[adam-farhi]

fix for #1991

[JacobBanghart]

Following up here based on the linked ticket. In /lib/pool_cluster.js the return of PoolCluster.of is this._namespaces[key] which is a class PoolNameSpace. PoolNameSpace should be what is returned in a .of but instead it returns a PoolCluster. A Promise-based version of this does not solve this issue you can see by cloning the repo linked in the ticket and installing your fork locally.

[JacobBanghart]

#2036 Adjusts it in the manner that I was describing. Sorry for leaving the issue out there and not getting back to it life gets hecktic.

[wellwelwel]

Closing in favor of #2084.