Allow easy configuration of Scheduler-Threads and name them more appropriate

[laeubi]

This is related to a new-post https://dev.eclipse.org/mhonarc/lists/jetty-users/msg07333.html

Jetty seem to execute AsyncListener.onTimeout() in a special Scheduler-Thread (one per connector and one for detecting LowResource-Conditions?). The Problem is, if any work in AsyncListener.onTimeout() takes longer (e.g. cancel a background task) then the timout-processing of the container is stopping completely. If you have an application that relies on timeout (e.g. long polling) this can slow down the whole processing.

Regardless of if this is considered "bad" it would be usefull to make it

1. possible to easy configure the number of Scheduler-Threads, currently this is hardcoded to 1 and you have to implement your own Scheduler and call the full blown ServerConnector cunstructor to change this
2. Make the naming of the threads more appropriate, currently they are named Scheduler-12345678, I would suggest something like Scheduler-http-8080, Scheduler-https-8433, Scheduler-lowresource or such, maybe appended with a number