Allow duration strings as --since/--until

Fixes #13107. This change enables Go duration strings
computed relative to the client machine’s time to be used
as input parameters to `docker events --since/--until`
and `docker logs --since` arguments.

Added unit tests for pkg/timeutils.GetTimestamp as well.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>