Introduce V2 API for PreKey updates and requests.

1) A /v2/keys controller.

2) Separate wire protocol PreKey POJOs from database PreKey
   objects.

3) Separate wire protocol PreKey submission and response POJOs.

4) Introduce a new update/response JSON format for /v2/keys.

src/main/java/org/whispersystems/textsecuregcm/entities/PreKeyBase.java

@@ -0,0 +1,8 @@
+package org.whispersystems.textsecuregcm.entities;
+
+public interface PreKeyBase {
+
+  public long getKeyId();
+  public String getPublicKey();
+
+}