Reduce use of scala.concurrent.duration.Duration

In most of the places we really are talking about a FiniteDuration,
hence use that class directly where it is an internal detail. Aside
from providing clarity, this also reduces potential confusion
with java.time.Duration.

Change-Id: I57d84c5ca058cfc6fa56ce57ebb0c8d4d3864a3a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>