LP#1879983: adjust available appointment times

LP#1879983: adjust available appointment times

This patch ensures that the first available appointment time
falls no sooner than two granularity intervals from the current
time. This is meant to give library staff enough time to stage
items.

The value of two step intervals is arbitrary and could be made configurable in
the future, though it does follow the hard-coding of the horizon in
fetch_to_be_staged().

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>