[Process] Use release branches to unblock PR merges

[avantgardnerio]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

In my understanding, at present:

1. Ballista allows git-refs to DataFusion and arrow
2. This allows features to be rapidly added to DataFusion, then Ballista features to be based on them (i.e. ListingSchemaProvider)
3. Except when a new DataFusion point release happens (i.e. v10)
4. In this case, merging of git-ref PRs is put on hold while a release of Ballista is prepared
5. After the Ballista release is live, then git-ref PRs can get merged again, and the cycle continues

The problem with this is that from step 3-5, there's a "mutex" on master and PRs are blocked, essentially "feature freeze".

Describe the solution you'd like

Rather than blocking multiple PRs from many contributors, it might be preferable to always leave Ballista master pointed at git-refs of DataFusion, and instead when an RC is declared to make a release branch on which the only work done is switching from git-ref back to release version of DataFusion and any accompanying compile fixes etc.

Describe alternatives you've considered

Keep doing what we're doing now.