Rework messages API to fail if you miss some deviceIds per number

src/main/java/org/whispersystems/textsecuregcm/util/Pair.java

@@ -1,10 +1,12 @@
 package org.whispersystems.textsecuregcm.util;
 
+import static com.google.common.base.Objects.equal;
+
 public class Pair<T1, T2> {
   private final T1 v1;
   private final T2 v2;
 
-  Pair(T1 v1, T2 v2) {
+  public Pair(T1 v1, T2 v2) {
     this.v1 = v1;
     this.v2 = v2;
   }
@@ -16,4 +18,14 @@ public class Pair<T1, T2> {
   public T2 second(){
     return v2;
   }
+
+  public boolean equals(Object o) {
+    return o instanceof Pair &&
+        equal(((Pair) o).first(), first()) &&
+        equal(((Pair) o).second(), second());
+  }
+
+  public int hashCode() {
+    return first().hashCode() ^ second().hashCode();
+  }
 }