Askrene algorithm switcher

[Lagrang3]

This PR only does a bit of refactoring to askrene.

I would like to add the possibility to switch algorithms in askrene.
This would not be available from the user interface, it is just for developers.
It would allow us to test different algorithms for experimenting performance and reliability
without risking breaking the existing code.
This PR sets the stage for switching between multiple MCF algorithms.

This PR is built on top of #7932.

[rustyrussell]

Not a bad idea! I'm just not sure about some of the refactorings.

[rustyrussell]

Handing arguments inside a struct (rather than using them explicitly) is an anti-pattern. We need to use this for callbacks, which is why we do it, but we try to constrain the damage to one location.

[Lagrang3]

Sorry I don't understand your request here.
I didn't change the arguments passed to do_getroutes.
That function changed from this:

static struct command_result *do_getroutes(struct command *cmd,
					   struct gossmap_localmods *localmods,
					   const struct getroutes_info *info)
{
	...
	// unpack getroutes_info and calls get_routes
	err = get_routes(cmd, cmd,
			 info->source, info->dest,
			 *info->amount, *info->maxfee, *info->finalcltv,
			 *info->maxdelay, info->layers, localmods, info->local_layer,
			 have_layer(info->layers, "auto.no_mpp_support"),
			 &routes, &amounts, info->additional_costs, &probability);

	// write results in a json stream and returns
	...	
	return command_finished(cmd, response);
}

static struct command_result *json_getroutes(struct command *cmd,
					     const char *buffer,
					     const jsmntok_t *params)
{
	// packages all input data into getroutes_info
	struct getroutes_info *info = tal(cmd, struct getroutes_info);
	...
	return do_getroutes(cmd, gossmap_localmods_new(cmd), info);
}

to this

static struct command_result *do_getroutes(struct command *cmd,
					   struct gossmap_localmods *localmods,
					   const struct getroutes_info *info)
{
        // do some stuff that previously was done in get_routes
	...
	// unpack getroutes_info and calls default_routes
       err = default_routes(rq, rq, srcnode, dstnode, *info->amount,
			     /* only one path? = */
			     have_layer(info->layers, "auto.no_mpp_support"),
			     *info->maxfee, *info->finalcltv, *info->maxdelay,
			     &flows, &probability);


	// write results in a json stream and returns
	...	
	return command_finished(cmd, response);
}

static struct command_result *json_getroutes(struct command *cmd,
					     const char *buffer,
					     const jsmntok_t *params)
{
	// packages all input data into getroutes_info
	struct getroutes_info *info = tal(cmd, struct getroutes_info);
	...
	return do_getroutes(cmd, gossmap_localmods_new(cmd), info);
}

Some of the code in the definition of get_routes was moved to mcf.c under the name
default_routes and some was moved to do_getroutes.

[rustyrussell]

OK, I broke this up into smaller commits for the refactoring, because I got very confused trying to read all that (as you can see, above)

Only minor changes:

1. apply_layers takes just the things it needs, not the entire getroutes_info.
2. convert_routes_to_flows returns the routes, and puts the amounts parameter at the end.
3. penultimate commit changes struct getroutes_info not to use pointers (that was only a side-effect of the param() weird calling convention, but it's not intuitive to have compulsory members be pointers).
4. Fixed an existing (minor) bug where the maxdelay parameter check was after the command_check_only test.

Done

[rustyrussell]

Fixed a commit missing a commit message, otherwise unchanged.

[rustyrussell]

Trivial rebase onto master now #7932 merged