Deliver nonpadding events instead of calling token removal functions.

This commit moves code that updates the state length and padding limit counts
out from the callback to its own function, for clarity.

It does not change functionality.

This commit moves the padding state limit checks and the padding rate limit
checks out of the token removal codepath, and causes all three functions to
get called from a single circpad_machine_count_nonpadding_sent() function.

It does not change functionality.

The code flow in theory can end up with a layer_hint to be NULL but in
practice it should never happen because with an origin circuit, we must have
the layer_hint.

Just in case, BUG() on it if we ever end up in this situation and recover by
closing the circuit.

Fixes #30467.

Signed-off-by: David Goulet <dgoulet@torproject.org>

Signed-off-by: David Goulet <dgoulet@torproject.org>

Signed-off-by: David Goulet <dgoulet@torproject.org>

See proposal 289 section 4.3 for more details.

It describes the flow control protocol at the circuit and stream level. If
there is no FlowCtrl protocol version, tor supports the unauthenticated flow
control features from its supported Relay protocols.

At this commit, relay will start advertising FlowCtrl=1 meaning they support
authenticated SENDMEs v1.

Closes #30363

Signed-off-by: David Goulet <dgoulet@torproject.org>

This reverts commit 7497c9193a0f2c891a0802bf5fbe73cf7ec1ca99.

This reverts commit ab8b80944967ee5a6a0c45dbf61839cf257bfe44.

- Move test-only cpath_get_n_hops() to crypt_path.c.
- Move onion_next_hop_in_cpath() and rename to cpath_get_next_non_open_hop().

The latter function was directly accessing cpath->state, and it's a first step
at hiding ->state.

Some of these functions are now public and cpath-specific so their name should
signify the fact they are part of the cpath module:

assert_cpath_layer_ok -> cpath_assert_layer_ok
assert_cpath_ok -> cpath_assert_ok
onion_append_hop -> cpath_append_hop
circuit_init_cpath_crypto -> cpath_init_circuit_crypto
circuit_free_cpath_node -> cpath_free
onion_append_to_cpath -> cpath_extend_linked_list

Now that we are using a constructor we should be more careful that we are
always using the constructor to initialize crypt_path_t, so make sure that
->private is initialized.

We are using an opaque pointer so the structure needs to be allocated on the
heap. This means we now need a constructor for crypt_path_t.

Also modify all places initializing a crypt_path_t to use the constructor.

Again everything is moved, apart from a free line using ->private.

Everything is moved, but the argument of the function is edited to access
->private->crypto.

Create some functions to eventually be able to hide crypt_path_t.crypto.