bump runtime-schema

[delivers #83638688]
[delivers #83638698]

Submodule src/github.com/cloudfoundry-incubator/runtime-schema d0bf31a..500886d:
  > use a worker pool for requesting lrp stops
  > complete tasks in a worker pool

Signed-off-by: Ashvin Agrawal <ashvin@pivotal.io>