Fix shadow samples in CI

[sfod]

Issue #, if available:

When multiple instances of shadow samples are running in CI, they use the same thing and interfere with each other causing crashes:

Received shadow document.
Shadow contains "color". Updating local value to "Shadow_Value_0"...

stderr:
terminate called after throwing an instance of 'std::future_error'
  what():  std::future_error: Promise already satisfied

Description of changes:

Handle multiple incoming shadow messages.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bretambrose]

Can't wait to delete these.