Fix incompatible cache between two versions of Tajine

For the moment ${sys_rootpath}/${download_checksum}/${_archive_rootdir}/var point to $var_path.

When Symfony starts, into var directory, it creates 2 directories we want persistent :

• sessions
• log

We can create a true var directory instead of link. Then, in the var directory, we create 2 links pointing in $var_path to sessions and log. The cache directory will be created when Symfony starts.

By this way, for a new version of Tajine, we will have a new cache directory.