Use ConcurrentHashMap to manage synchronization.

libraries/pushstore/impl/src/main/kotlin/io/element/android/libraries/pushstore/impl/DefaultUserPushStoreFactory.kt

@@ -26,6 +26,7 @@ import io.element.android.libraries.pushstore.api.UserPushStore
 import io.element.android.libraries.pushstore.api.UserPushStoreFactory
 import io.element.android.libraries.sessionstorage.api.observer.SessionListener
 import io.element.android.libraries.sessionstorage.api.observer.SessionObserver
+import java.util.concurrent.ConcurrentHashMap
 import javax.inject.Inject
 
 @SingleIn(AppScope::class)
@@ -39,15 +40,13 @@ class DefaultUserPushStoreFactory @Inject constructor(
     }
 
     // We can have only one class accessing a single data store, so keep a cache of them.
-    private val cache = mutableMapOf<SessionId, UserPushStore>()
+    private val cache = ConcurrentHashMap<SessionId, UserPushStore>()
     override fun create(userId: SessionId): UserPushStore {
-        return synchronized(cache) {
-            cache.getOrPut(userId) {
-                UserPushStoreDataStore(
-                    context = context,
-                    userId = userId
-                )
-            }
+        return cache.getOrPut(userId) {
+            UserPushStoreDataStore(
+                context = context,
+                userId = userId
+            )
         }
     }