mod_lua: Fix up LuaCodeCache:

- Check both mtime and size of a file when comparing with cache, in case the file is being written to while read
- If LuaCodeCache is 'never', only reload it if it has been run once or more.
- Never use cache if LuaScope is 'once'.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1367025 13f79535-47bb-0310-9956-ffa450edef68