WIP transaction based update
add pkgAcquire::TransactionHasError()
Download Release first, then Release.gpg

The old way of handling this was that pkgAcqMetaIndex was responsible
to check/move both Release and Release.gpg in place. This breaks
the assumption of the transaction that each pkgAcquire::Item has
a single File that its responsible for.
WIP cleanup pkgAcqMetaSig
Rework TransactionID stuff
make test_inreleae_to_release_reverts_all work
make test_unauthenticated_to_invalid_inrelease work
ensure InRelease->Release is transactional as well
fail early (again) on gpg sig failures
mve MetaKey into pkgAcqBaseIndex
fix transactionid passing
check hashes of compressed files as well
make errors more consistent
add gzip test and todo
make i-m-s work again
move clearsign check into pkgAcqMetaClearSig::Failed()
add ims check verify
really move clearsign check into pkgAcqMetaClearSig::Done()
cleanup
make compressed-indexes test pass again