add volume delete grpc API

fix https://github.com/chrislusf/seaweedfs/issues/808

weed/pb/volume_server.proto

@@ -33,6 +33,8 @@ service VolumeServer {
     }
     rpc VolumeUnmount (VolumeUnmountRequest) returns (VolumeUnmountResponse) {
     }
+    rpc VolumeDelete (VolumeDeleteRequest) returns (VolumeDeleteResponse) {
+    }
 
     // rpc VolumeUiPage (VolumeUiPageRequest) returns (VolumeUiPageResponse) {}
 
@@ -142,6 +144,12 @@ message VolumeUnmountRequest {
 message VolumeUnmountResponse {
 }
 
+message VolumeDeleteRequest {
+    uint32 volumd_id = 1;
+}
+message VolumeDeleteResponse {
+}
+
 message VolumeUiPageRequest {
 }
 message VolumeUiPageResponse {