Optimize delete_group

[megrogan]

Currently there is a long chain of c2c calls: user:delete_group -> group::c2c_delete_group -> group_index::c2c_delete_group -> local_group_index::c2c_delete_group

This means the original call from the client takes a long time to complete.

Also the group_index calls user::c2c_notify_group_deleted for each member of the group which is really slow cross-subnet.

This can surely be optimised...