Remove ewma setup code in init_circuit_base()/or_circuit_new() of circuitlist.c; it gets allocated when the circuit is attached to a circuitmux_t with that policy now

Implement circuitmux_get_first_active_circuit() in circuitmux.c; add pick_active_circuit() to circuitmux_policy_t in circuitmux.h

Implement circuitmux_notify_xmit_cells() and circuitmux_move_active_circ_to_tail() helper in circuitmux.c

Consistently allow policy alloc_cmux_data() and alloc_circ_data() functions to return NULL if the policy does not use this in circuitmux.c

Implement circuitmux_clear_policy(), circuitmux_get_policy() and circuitmux_set_policy() in circuitmux.c

Implement circuitmux_make_circuit_inactive(), circuitmux_make_circuit_active() and linked list helper functions in circuitmux.c

Implement circuitmux_is_circuit_active(), circuitmux_num_cells_for_circuit(), circuitmux_num_cells(), circuitmux_num_active_circuits() and circuitmux_num_circuits() in circuitmux.c

Implement circuitmux_clear_num_cells() and circuitmux_set_num_cells() in circuitmux.c, remove unneeded circuitmux_add_to_num_cells() from circuitmux.h

Implement circuitmux_attached_circuit_direction(), circuitmux_find_map_entry() and circuitmux_is_circuit_attached() in circuitmux.c

Refactor channel_write_cell()/channel_write_packed_cell()/channel_write_var_cell() to eliminate redundant code