calc_acting: Prefer up[0] as primary if possible

calc_acting: Prefer up[0] as primary if possible

Previously, we could get into a state where although up[0] has been
fully backfilled, acting[0] could be selected as a primary if it is able
to pull another peer into the acting set.  This also collects the logic
of choosing the best info into a helper function.

Signed-off-by: Samuel Just <samuel.just@dreamhost.com>