Fix ConcurrentModificationException in RecipePagingSourceFactory

It seems that it is possible to launch several coroutines on same
main thread of application. That's why it is possible to launch both
invoke and invalidate at the same time even though they are marked as
synchronized. To fix the issue this commit uses a concurrent collection
instead of synchronization.

app/src/test/java/gq/kirmanak/mealient/data/recipes/impl/RecipePagingSourceFactoryTest.kt

@@ -0,0 +1,35 @@
+package gq.kirmanak.mealient.data.recipes.impl
+
+import dagger.hilt.android.testing.HiltAndroidTest
+import gq.kirmanak.mealient.data.recipes.db.RecipeStorage
+import gq.kirmanak.mealient.test.HiltRobolectricTest
+import kotlinx.coroutines.*
+import org.junit.Before
+import org.junit.Test
+import javax.inject.Inject
+
+@ExperimentalCoroutinesApi
+@HiltAndroidTest
+class RecipePagingSourceFactoryTest : HiltRobolectricTest() {
+    @Inject
+    lateinit var storage: RecipeStorage
+    lateinit var subject: RecipePagingSourceFactory
+
+    @Before
+    fun setUp() {
+        subject = RecipePagingSourceFactory(storage)
+    }
+
+    @Test
+    fun `when modifying concurrently then doesn't throw`(): Unit = runBlocking {
+        (0..100).map {
+            async(Dispatchers.Default) {
+                for (i in 0..100) {
+                    subject.invalidate()
+                    subject.invoke()
+                }
+            }
+        }.awaitAll()
+    }
+
+}