scheduler: allowing task requests to be queued when max instances reached

previously maxtaskinstances count included queued requests
scheduler: on task start, cancel other queued requests for the same task

Otherwise such requests would stack if the task becomes longer than its
scheduling frequency.
This behaviour can be disabled with "(discardaliasesonstart none)"
todos review
compilation warning fix
scheduler: avoid stacking disabled task requests by canceling older ones
ui: removing dynamic task-related info from taskrequestsmodel

This data changes on a Task basis rather than a TaskRequest basis, and
therefore the TaskRequestsModel cannot be updated (or would be updated
at a high cost) when they change.
ui: fixed several recent ui bugs

bugs introduced with hot config reload and/or max instances on running
tasks rather than queued
http mean: fixing success status code on some network error cases

tasks was reported as successful when dns lookup fails or HTTP
connection times out