Help needed with v4.0 release

[bocharsky-bw]

Currently, master branch contains a lot of unreleased changes that blocks us to make new releases with some required changes that were recently merged into master. I'm not sure I completely understand when it will be ready for v4.0 release and what else needed to be done here. Probably @odolbeau will be able to finish his work here (referencing to #36 #38 #39 PRs) or at least help and supervise about what else should be done here so we finally could release v4.0.

For now, I suppose we need to merge new PRs in both branches, and probably merge to 3.x branch first which will be merged upstream into master then. That's how I understand our strategy should be so far, though not sure how much conflicts it will bring. At least, IIRC that's the exact Symfony strategy to support a few branches.

Thereby, probably we should temporarily make 3.x branch as the main branch here? So all the new PRs will be targeting to it instead of master? Because it's easy to miss things on review and merge in incorrect branch. If I understand correctly, we can't merge into master first, and then merge master into 3.x branch, i.e. it should be done in the reverse order.

Any thoughts about it?