HTTP Workers: use runInterruptible instead of interrupting manually (bitfireAT/davx5#444)

* RefreshCollectionsWorker: use runInterruptible instead of interrupting manually

* SyncWorker: use CoroutineWorker + runInterruptible

* Use global SyncWorkDispatcher that guarantees classLoader to be set

* Set SyncWorkDispatcher for whole SyncWorker's doWork

* Remove obsolete test

* SyncManager: add structured concurrency again

* Use up to <number of processors> threads for synchronization

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Co-authored-by: Sunik Kupfer <kupfer@bitfire.at>