acme: deprecate state_dir

acme: deprecate state_dir

state_dir is actually a hardcoded value in conffiles. Allowing users to
customize it could result in losing certificates after upgrading if they
don't also specify the dir as being preserved. We shouldn't default to
this dangerous behavior.

With the new ACME package, certificates live in the standard location
/etc/ssl/acme, users who need to do certificate customizations should
look for them in that dir instead.

Signed-off-by: Glen Huang <i@glenhuang.com>