The initial implementation of spawn-ready didn't properly compose over
MakeService instances. This introduces `MakeSpawnReady` as a factory
type for SpawnReady, and changes Layer to produce `MakeSpawnReady`
instances.
Some layers cannot guarantee that they will poll inner services in a
timely fashion. For instance, the balancer polls its inner services to
check for readiness, but it does so randomly. If its inner service
must be polled several times to become ready, e.g., because it's driving
the initiation of a TLS connection, then the balancer may not drive the
handshake to completion.
The `SpawnReady` layer ensures that its inner service is driven to
readiness by spawning a background task.
Oliver Gould