Add thresholds to each request

[JavierUhagon]

This PR adds an argument to each request to add a threshold to it, if a request response time is over the threshold it'll be logged

This also adds a new stat in the webui (the threshold) that's hidden if there are no thresholds defined.

Images

Related issue: #2668

I want to know if @cyberw thinks this can be added to the core (and it's being done nicely) before I resume my work on it!