storage_service: Fix ignored future in on_alive
storage_service: Fix ignored future in on_alive

This patch ensures the future created by invoke_on_all is not ignored
by waiting on it, which is safe to do since we are within a
seastar::async context.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <1462989837-7326-1-git-send-email-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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