Boostrap persistent state for volume servers. (#7984)

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.

weed/pb/master.proto

@@ -4,6 +4,8 @@ package master_pb;
 
 option go_package = "github.com/seaweedfs/seaweedfs/weed/pb/master_pb";
 
+import "volume_server.proto";
+
 //////////////////////////////////////////////////
 
 service Seaweed {
@@ -84,6 +86,9 @@ message Heartbeat {
   uint32 grpc_port = 20;
   repeated string location_uuids = 21;
   string id = 22; // volume server id, independent of ip:port for stable identification
+
+  // state flags
+  volume_server_pb.VolumeServerState state = 23;
 }
 
 message HeartbeatResponse {