dex: implement an execution circuit-breaker

[erwanor]

Is your feature request related to a problem? Please describe.
In the DEX design, we put a good amount of effort into ensuring we always make forward progress. But forward progress could be very slow. We don’t think that it will have pathological behavior – but it’s possible we could have missed something that then turns into a DoS vector. Similarly to the #2986, it could be useful to have some kind of “execution breaker” that will bail out with incomplete fill after some execution limit. This means that if the DEX engine does have a DoS vector, it does not affect users of the shielded pool.

Describe the solution you'd like
We need to implement a DEX-engine specific metering system that tracks the number of operations performed during routing execution e.g. filling against a position, or assembling a FrontierTx and concludes execution if a threshold is exceeded. The threshold could be a DEX chain parameter.

[erwanor]

Here we should have two levels of tracking:

• one that limits the number of positions
• one that controls the amount of work that can be expanded on path search