util_filter: protect ap_filter_t private fields from external (ab)use.

Introduce opaque struct ap_filter_private to move ap_filter_t "pending", "bb"
and "deferred_pool" fields to the "priv" side of things.

This allows to trust values set internally (only!) in util_filter code, and
make useful assertions between the different functions calls, along with the
usual nice extensibility property.

Likewise, the private struct ap_filter_conn_ctx in conn_rec (from r1839997)
allows now to implement the new ap_acquire_brigade() and ap_release_brigade()
functions useful to get a brigade with c->pool's lifetime. They obsolete
ap_reuse_brigade_from_pool() which is replaced where previously used.

Some comments added in ap_request_core_filter() regarding the lifetime of the
data it plays with, up to EOR...

MAJOR bumped (once again).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1840149 13f79535-47bb-0310-9956-ffa450edef68