gossip: Decouple feature lifetime from the gossiper
gossip: Decouple feature lifetime from the gossiper

This patch changes the gms::feature destructor so it
checks whether the gossiper has been stopped before trying
to unregister the feature.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla