prop224: Specialize interface of hs_circuitmap_get_rend_circ_client_side()

We currently use hs_circuitmap_get_rend_circ_client_side() for two reasons: a) To proceed with the rend protocol as a client when we receive an intro ack (in handle_introduce_ack_success()). b) To close useless rend circuits in close_or_reextend_intro_circ().

To fit these two scenarios, the function hs_circuitmap_get_rend_circ_client_side() currently returns all sorts of rend circs (established and unestablished).

We can improve the logic and semantics here by splitting into two funcs. One that returns only established circs (used for (a)), and another that retuns all kinds of circs (used for (b)).