Move main development back to `master`-branch

[nknapp]

Currently, we are developing on the 4.x branch, but everything has to be merged into master to keep it maintainable. This is not a lot of work, but a little bit frustrating. I have taken the time and searched through both logs. The main differences (i.e. commits that are part of master and not 4.x are):

Breaking changes:

• Remove stringParams and trackIds mode 3f77b82, 118836f
• Remove semi-documented _setup and _child APIs, 63a0889
• Remove amd files from the build, 8517352, 5302782
• Remove support for Node <= 12 (no support for EOL versions)

Bugfixes

• Use objects for hash value tracking, 768ddbd

Everything else if chore or docs. @wycats keeping these commits on master would require us to bump to 5.0. I would probably remove support for lambdas (#1565) as well, when doing that.

[rwjblue]

FWIW, I believe that the divergence between master and 4.x are partially to blame for the ergonomic regressions reported in #1562 (specific explanation in #1562 (comment)).

[nknapp]

Might be, yes I'm not 100% sure, but this is the only explanation I have

[marado]

AFAICS, #1274 is also only merged on 5.0 but not on 4.x.

[nknapp]

Yes, that was before I told everyone to point PRs to the 4.x branch.

[ErisDS]

The master branch now also uses yargs instead of optimist/minimist, and has dropped support for Node.js < v10

[nknapp]

This is an implicit goal for 5.0, but the issue description documents parts of the changelog for 5.0, so I am assigning it to the milestone