[BZ 955816] Ask server for last known metric val if not known otherwise.
[BZ 955816] Don't update the cache with the retrieved value

This could lead to concurrent update exceptions and is not worth
dealing with in some async manner given this is an edge case.
[BZ 955816] Added API difference record to client-api.
Merge pull request #116 from metlos/bug/955816

[BZ 955816] Ask server for last known trait val if not known otherwise.