feature: allow users to provide custom pool class

[callmehiphop]

I think no one is a stranger to the fact that we've been seeing a fair amount of chatter around issues with our pooling implementation. Its come to my attention that some teams might want to opt out of certain features of our Pooling class or in some cases just have a finer level of control. I would like to propose accepting a Pool constructor that could be used in place of our Pooling class.

I've reviewed our code and with a light refactor I think the bare minimum could be the following interface

interface PoolInterface extends EventEmitter {
  new (database: Database);
  open();
  close(callback: (err: Error) => void);
  getReadSession(callback: (err: Error, session: Session) => void);
  getwriteSession(callback: (err: Error, session: Session, transaction: Transaction) => void);
  release(session: Session);
}

Integrating would be as simple as

class CustomPool extends EventEmitter {
  // ...
}

const database = instance.database('my-db', CustomPool);

Thoughts?

[callmehiphop]

/cc @googleapis/yoshi-nodejs @anup-honey @walshie4