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/PreKeyResponseV1.java

@@ -0,0 +1,70 @@
+/**
+ * Copyright (C) 2014 Open WhisperSystems
+ *
+ * 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.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+
+public class PreKeyResponseV1 {
+
+  @JsonProperty
+  @NotNull
+  @Valid
+  private List<PreKeyV1> keys;
+
+  @VisibleForTesting
+  public PreKeyResponseV1() {}
+
+  public PreKeyResponseV1(PreKeyV1 preKey) {
+    this.keys = new LinkedList<>();
+    this.keys.add(preKey);
+  }
+
+  public PreKeyResponseV1(List<PreKeyV1> preKeys) {
+    this.keys = preKeys;
+  }
+
+  public List<PreKeyV1> getKeys() {
+    return keys;
+  }
+
+  @VisibleForTesting
+  public boolean equals(Object o) {
+    if (!(o instanceof PreKeyResponseV1) ||
+        ((PreKeyResponseV1) o).keys.size() != keys.size())
+      return false;
+    Iterator<PreKeyV1> otherKeys = ((PreKeyResponseV1) o).keys.iterator();
+    for (PreKeyV1 key : keys) {
+      if (!otherKeys.next().equals(key))
+        return false;
+    }
+    return true;
+  }
+
+  public int hashCode() {
+    int ret = 0xFBA4C795 * keys.size();
+    for (PreKeyV1 key : keys)
+      ret ^= key.getPublicKey().hashCode();
+    return ret;
+  }
+}