gms: Add is_shutdown helper for endpoint_state class
gms: Add is_shutdown helper for endpoint_state class

It will be used by streaming manager to check if a node is in shutdown
status.

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla