The function tablet_map::get_secondary_replica() is used by Alternator
TTL to choose a node different from get_primary_replica(). Unfortunately,
recently (commits 817fdad and d88037d) the implementation of the latter
function changed, without changing the former. So this patch changes
the former to match.
The next two patches will have two tests that fail before this patch,
and pass with it:
1. A unit test that checks that get_secondary_replica() returns a
different node than get_primary_replica().
2. An Alternator TTL test that checks that when a node is down,
expirations still happen because the secondary replica takes over
the primary replica's work.
Fixes SCYLLADB-777
Signed-off-by: Nadav Har'El <nyh@scylladb.com>