feat(pool): add a Singleton pool type (#226)

This creates a `Singleton` pool service which wraps an inner make-service, and queues up all calls while waiting for the one instance to be created, and then hands out clones. This pattern fits HTTP/2 well.

The current implementation mirrors much of how the legacy pool handles connecting with HTTP/2 prior knowledge.

Closes https://github.com/hyperium/hyper/issues/3953