core: always allocate filters (ap_filter_t) on f->c->pool.

When filters are allocated on f->r->pool, they may be destroyed any time
underneath themselves which makes it hard for them to be passed the EOR and
forward it (*f can't be dereferenced anymore when the EOR is destroyed, thus
before request filters return).

On the util_filter side, it also makes it impossible to flush pending request
filters when they have set aside the EOR, since f->bb can't be accessed after
it's passed to the f->next.

So we always use f->c->pool to allocate filters and pending brigades, and to
avoid leaks with keepalive requests (long living connections handling multiple
requests), filters and brigades are recycled with a cleanup on f->r->pool.

Recycling is done (generically) with a spare data ring (void pointers), and a
filter(s) context struct is associated with the conn_rec to maintain the rings
by connection, that is:

    struct ap_filter_conn_ctx {
        struct ap_filter_ring *pending_input_filters;
        struct ap_filter_ring *pending_output_filters;

        struct ap_filter_spare_ring *spare_containers,
                                    *spare_brigades,
                                    *spare_filters,
                                    *spare_flushes;
        int flushing;
    };

MMN major bumped (again).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1839997 13f79535-47bb-0310-9956-ffa450edef68