Initial multi device support refactoring.

1) Store account data as a json type, which includes all
   devices in a single object.

2) Simplify message delivery logic.

3) Make federated calls a pass through to standard controllers.

4) Simplify key retrieval logic.

src/main/java/org/whispersystems/textsecuregcm/push/TransientPushFailureException.java

@@ -0,0 +1,11 @@
+package org.whispersystems.textsecuregcm.push;
+
+public class TransientPushFailureException extends Exception {
+  public TransientPushFailureException(String s) {
+    super(s);
+  }
+
+  public TransientPushFailureException(Exception e) {
+    super(e);
+  }
+}