broker: there is no way to terminate a groups.get streaming RPC

[garlick]

Problem: although there is a disconnect handler that does it, there is currently no way to terminate a streaming groups.get RPC if you want to continue running. This would be useful for the bootstrap mechanism proposed in #5184.

Edit: duh I just realized this is not needed in #5184 since in that case, the remote broker handle is closed after bootstrap is complete. So maybe for the back burner.