Always use bluebird as the Promise implementation

We are using some of the non-es6 features like .delay() and promisify. Make
sure that those are consistently available even on 0.11.

It also turns out that bluebird is still faster than the built-in Promise
implementation in 0.11 (just re-ran the benchmarks).
Improve table creation retries

On the cluster, schema agreement takes longer than on a single box. In tests
this led to some concurrent table creations failing in the first attempt. This
patch addresses the issue by trying harder & backing off.

- Retry 100 times instead of just 10
- Back off exponentially with a random component