WIP: Release timeline for 2.15.1, 2.16. and 2.17

[Roland McGrath]

> I would like the release process to be uniform for major or minor, it
> shouldn't matter.

Well, there are some important differences.  

A minor release must never perturb the ABI.  So it should be a
requirement that check-abi has no regressions with the existing
abilist files.  Conversely, for a major release it should be a
requirement to make sure abilist files have been updated as necessary
so check-abi passes and to manually ensure that the abilist changes
are kosher (only add a new symbol version set, don't change existing ones).

And, of course, the whole schedule thing is entirely different for the
two kinds of release.


Thanks,
Roland