DEBUG: add timeout to realm retrieval

This is a TEMPORARY measure to hopefully help diagnose deadlocks of
`OnlineCachingAssetStore` on single-thread CI runs.

I am unable to reproduce those deadlocks locally despite almost an hour
of debug test runs in various configurations. I can reproduce *other*
tests deadlocking, but on nothing seemingly related to this one new
class.

Author: bdach

osu.Game/Database/RealmAccess.cs

@@ -778,7 +778,7 @@ private Realm getRealmInstance()
                 // Ensure that the thread that currently has the `realmRetrievalLock` can retrieve nested contexts and not deadlock on itself.
                 if (!currentThreadHasRealmRetrievalLock.Value)
                 {
-                    realmRetrievalLock.Wait();
+                    realmRetrievalLock.Wait(10000);
                     currentThreadHasRealmRetrievalLock.Value = true;
                     tookSemaphoreLock = true;
                 }