-> this happens if the processing of items is slower as the fetching of
new
-> if a big library is synced, the old behavior could lead to extensive
use of memory
-> the semaphore acts like a buffer that only allows fetching of new
items from the library if old ones are processed
-> the current size of the 'buffer' is hard coded to 2 * [max. item
fetch limit] * [number of download threads]
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