MINOR: filters: Extract proxy stuff from the struct filter

MINOR: filters: Extract proxy stuff from the struct filter

Now, filter's configuration (.id, .conf and .ops fields) is stored in the
structure 'flt_conf'. So proxies own a flt_conf list instead of a filter
list. When a filter is attached to a stream, it gets a pointer on its
configuration. This avoids mixing the filter's context (owns by a stream) and
its configuration (owns by a proxy). It also saves 2 pointers per filter
instance.