core: Rework archive-z mode to have header be uncompressed

This is an incompatible change to archive-z, thus it is now renamed to
archive-z2 and ostree will no longer parse archive-z.

I noticed in perf that we were spending some time zlib-decompressing
file headers, which is just inefficient.  Rather than do this, keep
the headers uncompressed, and just zlib-compress content.

tests/t0002-archivez.sh

@@ -23,7 +23,7 @@ set -e
 
 echo '1..11'
 
-setup_test_repository "archive-z"
+setup_test_repository "archive-z2"
 echo "ok setup"
 
 . ${SRCDIR}/archive-test.sh

tests/t0011-pull-archive-z.sh

@@ -23,6 +23,6 @@ set -e
 
 echo '1..2'
 
-setup_fake_remote_repo1 "archive-z"
+setup_fake_remote_repo1 "archive-z2"
 
 . ${SRCDIR}/pull-test.sh