No longer support unreleased STARR.

[Best viewed per-commit. The first commit just moves some xml around.]

The STARR ("stable reference") compiler is used to bootstrap the compiler.
It is now always resolved from maven, based on the `starr.version`
property (stored in `versions.properties`).

Before, we used the `.desired.sha1` mechanism to pull a set of jars
that define the compiler used to build locker ("local reference"),
which then builds quick.

From now on, we only support officially released versions of STARR.
Milestones are allowed of course, which means that, instead of
breaking change, STARR evolution must support old and new behavior
for at least one milestone cycle.

For local development, use the `replacestarr` target as before.
It builds quick (core only) and publishes it to your local maven repo
with a generated version number, which is saved as `starr.version`
in `build.properties` for convenience (overriding `versions.properties`),
so that your next build will use this version of the compiler for STARR.

You may now think of STARR as STAble Reference Release -- if you will.

Review by @gkossakowski /cc @xeno-by, @paulp, @retronym, @jamesiry.