Use buffers object to check wether a section has already been loaded

[Evert-R]

This PR fixes the bug that occurs when you play matching_pairs_lite for the second time (via the dashboard) with 8 originals.

When the sections are loaded the buffer list gets cleared before loading the first section.
we used previousSource to determine wether a section had already been loaded.
A section might still be loaded, but if it isn't added to the buffers object, it will not be passed on to matching pairs.

I now changed this it to checking the buffers object to check if a section has been loaded.

Fixes: #1368

[BeritJanssen]

As discussed IRL, make sure this change doesn't introduce a loading time in Hooked experiments, for which the check against previousSource was introduced. Perhaps the store slice for actions added per #1397 can be the basis of a more robust solution.

[Evert-R]

As discussed IRL, make sure this change doesn't introduce a loading time in Hooked experiments, for which the check against previousSource was introduced. Perhaps the store slice for actions added per #1397 can be the basis of a more robust solution.

I made some changes for previously loaded single section requests.