shell: add volumeServer.leave command

weed/pb/volume_server.proto

@@ -85,6 +85,8 @@ service VolumeServer {
 
     rpc VolumeServerStatus (VolumeServerStatusRequest) returns (VolumeServerStatusResponse) {
     }
+    rpc VolumeServerLeave (VolumeServerLeaveRequest) returns (VolumeServerLeaveResponse) {
+    }
 
     // <experimental> query
     rpc Query (QueryRequest) returns (stream QueriedStripe) {
@@ -425,6 +427,11 @@ message VolumeServerStatusResponse {
     MemStatus memory_status = 2;
 }
 
+message VolumeServerLeaveRequest {
+}
+message VolumeServerLeaveResponse {
+}
+
 // select on volume servers
 message QueryRequest {
     repeated string selections = 1;