use multicall for performance

[ktdreyer]

As we add more features to koji_tag, we're making more and more XML-RPC calls, and this is slowing down the playbook operation.

We should be able to get all the information we need about the tag in one single "read" multicall, and then we can make all the changes we need in another single "write" multicall.

[mizdebsk]

It is probably not possible to do everything it just two multicall calls. For example, some calls require passing tag ID, but you don't know that until tag is created. But it should be possible to group everything into a few RPC calls.

[ktdreyer]

I played around with multicall recently and improved the upstream docs at https://pagure.io/koji/pull-request/3226