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/PreKeyStateV1.java

@@ -0,0 +1,55 @@
+/**
+ * Copyright (C) 2014 Open Whisper Systems
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.whispersystems.textsecuregcm.entities;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.annotations.VisibleForTesting;
+
+import javax.validation.Valid;
+import javax.validation.constraints.NotNull;
+import java.util.List;
+
+public class PreKeyStateV1 {
+
+  @JsonProperty
+  @NotNull
+  @Valid
+  private PreKeyV1 lastResortKey;
+
+  @JsonProperty
+  @NotNull
+  @Valid
+  private List<PreKeyV1> keys;
+
+  public List<PreKeyV1> getKeys() {
+    return keys;
+  }
+
+  @VisibleForTesting
+  public void setKeys(List<PreKeyV1> keys) {
+    this.keys = keys;
+  }
+
+  public PreKeyV1 getLastResortKey() {
+    return lastResortKey;
+  }
+
+  @VisibleForTesting
+  public void setLastResortKey(PreKeyV1 lastResortKey) {
+    this.lastResortKey = lastResortKey;
+  }
+}