db: tracking builds & packages backend.

db: tracking builds & packages backend.

note: database needs to be completely recreated with this commit.

packages are flagged for build iff:
- there is no latest build
- the database (or cran2deb) has changed since the last build
- the debian epoch for cran2deb has changed since the last build
- the R version of the last build does not match that of the very
latest R package.
it should be easy to add further conditions in as this is all done with
one moderately-sized SQL SELECT.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/cran2deb@89 edb9625f-4e0d-4859-8d74-9fd3b1da38cb