Add volume server RPCs to read and update state flags. (#8186)

* 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.

* Add volume server RPCs to read and update state flags.

weed/pb/volume_server.proto

@@ -53,7 +53,11 @@ service VolumeServer {
     }
     rpc VolumeStatus (VolumeStatusRequest) returns (VolumeStatusResponse) {
     }
-    // TODO(issues/7977): add RPCs to control state flags
+
+    rpc GetState (GetStateRequest) returns (GetStateResponse) {
+    }
+    rpc SetState (SetStateRequest) returns (SetStateResponse) {
+    }
 
     // copy the .idx .dat files, and mount this volume
     rpc VolumeCopy (VolumeCopyRequest) returns (stream VolumeCopyResponse) {
@@ -270,6 +274,21 @@ message VolumeStatusResponse {
     uint64 file_deleted_count = 4;
 }
 
+message GetStateRequest {
+}
+message GetStateResponse {
+    VolumeServerState state = 1;
+}
+
+message SetStateRequest {
+    // SetState updates *all* volume server flags at once. Retrieve state with GetState(),
+    // modify individual flags as required, then call this RPC to update.
+    VolumeServerState state = 1;
+}
+message SetStateResponse {
+    VolumeServerState state = 1;
+}
+
 message VolumeCopyRequest {
     uint32 volume_id = 1;
     string collection = 2;