Move database calls from Schedulers.io to Schedulers.computation (#8374)

### Description

Move database calls from `Schedulers.io` to `Schedulers.computation`.
The io scheduler is unbounded, starting an arbitrary number of threads.
This might be a reason for the lock inflation failing. In contrast, the
`compute` scheduler is limited by the number of available threads.

Contributes to #8338

### Checklist
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net/discovery/src/main/java/de/danoeh/antennapod/net/discovery/FyydPodcastSearcher.java

@@ -32,7 +32,7 @@ public class FyydPodcastSearcher implements PodcastSearcher {
 
             subscriber.onSuccess(searchResults);
         })
-                .subscribeOn(Schedulers.io())
+                .subscribeOn(Schedulers.computation())
                 .observeOn(AndroidSchedulers.mainThread());
     }
 

net/download/service/src/main/java/de/danoeh/antennapod/net/download/service/feed/DownloadServiceInterfaceImpl.java

@@ -78,8 +78,8 @@ public class DownloadServiceInterfaceImpl extends DownloadServiceInterface {
         String tag = WORK_TAG_EPISODE_URL + media.getDownloadUrl();
         Future<List<WorkInfo>> future = WorkManager.getInstance(context).getWorkInfosByTag(tag);
         Observable.fromFuture(future)
-                .subscribeOn(Schedulers.io())
-                .observeOn(Schedulers.io())
+                .subscribeOn(Schedulers.computation())
+                .observeOn(Schedulers.computation())
                 .subscribe(
                     workInfos -> {
                         for (WorkInfo info : workInfos) {

net/sync/service/src/main/java/de/danoeh/antennapod/net/sync/service/LockingAsyncExecutor.java

@@ -28,7 +28,7 @@ public class LockingAsyncExecutor {
                 } finally {
                     lock.unlock();
                 }
-            }).subscribeOn(Schedulers.io())
+            }).subscribeOn(Schedulers.computation())
                     .subscribe();
         }
     }