lib/pull: Add support for timestamp-check option, use in upgrader

lib/pull: Add support for timestamp-check option, use in upgrader

For both flatpak and ostree-as-host, we really want to verify up front during
pulls that we're not being downgraded. Currently both flatpak and
`OstreeSysrootUpgrader` do this before deployments, but at that point we've
already downloaded all the data, which is annoying.

Closes: https://github.com/ostreedev/ostree/issues/687
Closes: #1055
Approved by: jlebon