Remove duplication in java builds of fjbg/asm/forkjoin
Preparation for faster PR validation

Added starr.number, so that `ant -propertyfile starr.number -Dlocker.skip=1`
works across branches.

Introduced target `test.core`, which skips `test.stability`, since that requires
`locker == quick`, whereas `locker == starr` with the above settings.
(It's interesting to see for which files it fails when that assumption is broken.)

Stability will still be tested on a nightly basis. It's rarely broken.
Let continuations library sources determine docs.lib's actuality