Call the shutdown drain maintenance mode

A node draining before a shutdown is in maintenance mode: /v1/ready reports "maintenance": true (was "draining"), and the 503 says the node is in maintenance mode. Maintenance is a runtime state next to the configured role, not a role: a later admin command can enter and leave it without a shutdown, with the same semantics. server_drain.go is now server_maintenance.go.

server/server.yml

@@ -89,9 +89,10 @@
 #   balancer agents can route to active nodes only.
 # - experimental-cluster-drain-duration is how long an active node takes, on shutdown (SIGTERM),
 #   to close its subscriber connections, so they reconnect to the other nodes gradually instead
-#   of all at once. While draining, new subscriptions get 503 and /v1/ready fails, so the load
-#   balancer must retry on 503. A node drains only if another active node is up. Make sure the
-#   service manager waits long enough (systemd: TimeoutStopSec, default 90s). Set to 0 to disable.
+#   of all at once. It is in maintenance mode meanwhile: new subscriptions get 503 and /v1/ready
+#   fails, so the load balancer must retry on 503. A node drains only if another active node is
+#   up. Make sure the service manager waits long enough (systemd: TimeoutStopSec, default 90s).
+#   Set to 0 to disable.
 #
 # Clustering also requires "database-url": the shared PostgreSQL database carries the durable
 # state and the cluster's own coordination tables.