Reorganizing the squid builds

Hi all,
  Now that the matrix jobs feature has matured in jenkins, I'd like to
restructure the jobs we use for our continuous integration testing to
reduce their number.

I propose to use these jobs:
- X.Y-matrix -> gcc-only (for now), build a specific revision
- trunk-compilers-matrix -> uses a combination filter to test on any
unix-like platform using any available compiler (gcc, clang, icc, msvc
to come as soon as the windows port reaches stability)
- anybranch-wholefarm-matrix -> same as above, but with some
parameters (a branch to be used, whether to use ccache or not, and an
address to be notified).

Ideally trunk-compilers-matrix should replace most 3.HEAD-* jobs,
while anybranch-wholefarm-matrix can replace most experimental jobs
(after all, a patch is just a push to lp away). The only nonmatrix
jobs that would remain are
- coverity-submit
- mswin
- possibly some support jobs

What do you think?
I plan to go ahead on Aug 25th, unless there are no objections.

Thanks!

-- 
    /kinkie