Add job to clean up early message receipts.

app/src/main/java/org/thoughtcrime/securesms/database/model/ServiceMessageId.kt

@@ -0,0 +1,16 @@
+package org.thoughtcrime.securesms.database.model
+
+import org.thoughtcrime.securesms.recipients.RecipientId
+
+/**
+ * Represents the messages "ID" from the service's perspective, which identifies messages via a
+ * a (sender, timestamp) pair.
+ */
+data class ServiceMessageId(
+  val sender: RecipientId,
+  val sentTimestamp: Long
+) {
+  override fun toString(): String {
+    return "MessageId($sender, $sentTimestamp)"
+  }
+}