fix(controller): give processes unique counters

Previously, when a container was created through App.scale(),
it would give the container a number based off the highest
container number amongst all containers available for the application.
This has been changed to give the container a number based off the
highest container number amongst all containers with the same process
type.

fixes #769
Merge pull request #890 from deis/769-unique-process-counter

fix(controller): give processes unique counters