Block RPC write operations on volume servers when maintenance mode is enabled (#8115)

* Boostrap persistent state for volume servers.

This PR implements logic load/save persistent state information for storages
associated with volume servers, and reporting state changes back to masters
via heartbeat messages.

More work ensues!

See https://github.com/seaweedfs/seaweedfs/issues/7977 for details.

* Block RPC operations writing to volume servers when maintenance mode is on.

weed/server/volume_server_test.go

@@ -0,0 +1,72 @@
+package weed_server
+
+import (
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb/volume_server_pb"
+	"github.com/seaweedfs/seaweedfs/weed/storage"
+)
+
+func TestMaintenanceMode(t *testing.T) {
+	testCases := []struct {
+		name         string
+		pb           *volume_server_pb.VolumeServerState
+		want         bool
+		wantCheckErr string
+	}{
+		{
+			name:         "non-initialized state",
+			pb:           nil,
+			want:         false,
+			wantCheckErr: "",
+		},
+		{
+			name: "maintenance mode disabled",
+			pb: &volume_server_pb.VolumeServerState{
+				Maintenance: false,
+			},
+			want:         false,
+			wantCheckErr: "",
+		},
+		{
+			name: "maintenance mode enabled",
+			pb: &volume_server_pb.VolumeServerState{
+				Maintenance: true,
+			},
+			want:         true,
+			wantCheckErr: "volume server test_1234 is in maintenance mode",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			vs := VolumeServer{
+				store: &storage.Store{
+					Id: "test_1234",
+					State: &storage.State{
+						FilePath: "/some/path.pb",
+						Pb:       tc.pb,
+					},
+				},
+			}
+
+			if got, want := vs.MaintenanceMode(), tc.want; got != want {
+				t.Errorf("MaintenanceMode() returned %v, want %v", got, want)
+			}
+
+			err, wantErrStr := vs.CheckMaintenanceMode(), tc.wantCheckErr
+			if err != nil {
+				if wantErrStr == "" {
+					t.Errorf("CheckMaintenanceMode() returned error %v, want nil", err)
+				}
+				if errStr := err.Error(); errStr != wantErrStr {
+					t.Errorf("CheckMaintenanceMode() returned error %q, want %q", errStr, wantErrStr)
+				}
+			} else {
+				if wantErrStr != "" {
+					t.Errorf("CheckMaintenanceMode() returned no error, want %q", wantErrStr)
+				}
+			}
+		})
+	}
+}