MINOR: cache: Improve and simplify the cache configuration check

MINOR: cache: Improve and simplify the cache configuration check

To do so, a dedicated configuration has been added on cache filters. Before the
cache filter configuration pointed directly to the cache it used. Now, it is the
dedicated structure cache_flt_conf. Store and use rules also point to this
structure. It is linked to the cache the filter must used. It also contains a
flags field. This will allow us to define the behavior of a cache filter when a
response is stored in the cache or delivered from it.

And now, Store and use rules uses a common parsing function. So if it does not
already exists, a filter is always created for both kind of rules. The cache
filters configuration is checked using their check callback. In the postparser
function, we only check the caches configuration. This removes the loop on all
proxies in the postparser function.