As described in tower-rs#286, `Balance` had a few problems:
- it is responsible for driving all inner services to readiness, making
  its `poll_ready` O(n) and not O(1);
- the `choose` abstraction was a hinderance. If a round-robin balancer
  is needed it can be implemented separately without much duplicate
  code; and
- endpoint errors were considered fatal to the balancer.

This changes replaces `Balance` with `P2CBalance` and removes the
`choose` module.

Endpoint service failures now cause the service to be removed from the
balancer gracefully.

Endpoint selection is now effectively constant time, though it biases
for availability in the case when random selection does not yield an
available endpoint.

`tower-test` had to be updated so that a mocked service could fail after
advertising readiness.

The balance layer now operates over MakeP2CBalance, which composes over
layers that produce Discover instances.