The `circuitmux_find_map_entry()` function is currently taking more than 3% of the CPU on a busy relay. It is literally the second highest after curv25519 crypto stuff (which is expected to be high). We can't really optimize that function so much but we can try to call it less! For instance, at every single cell we append to a circuit in `append_cell_to_circuit_queue()` we call `update_circuit_on_cmux()` which calls *4* functions in succession that calls `circuitmux_find_map_entry()`: * `circuitmux_is_circuit_attached()` * `circuitmux_attached_circuit_direction()` * `circuitmux_set_num_cells()` and this function can call it again with `circuitmux_make_circuit_inactive()` or `circuitmux_make_circuit_active()`. This is quite a lot of CPU at _each_ cell especially when we do have many many circuits.