[MAJOR] separate sess->proxy into sess->{fe,fi,be}

[MAJOR] separate sess->proxy into sess->{fe,fi,be}

The references to the proxy from the session have been turned into
Frontend (fe), Filters (fi) and Backend (be). This should ease the
migration to the L7 switching features. Next step will be to kill
the struct proxy and have 3 independant structs instead, each
referenced from entities called listener, frontend, filters and
backend.