Speed up sending request to ksysguardd

Summary:
While investigating bug 407526, I discovered that most of the time is
spent in SensorAgent::sendRequest.
This is because it searches the whole queue lineraly, which could take a
lot of time if there's a lot of requests. This patch speeds it up by
using a QSet of outstanding requests, obviating the need to search the
queues.

Part of T11473: #407526: plasma + ksysguardd causing high cpu load

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23595