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/PreKeyStateV2.java

@@ -0,0 +1,74 @@
+/**
+ * 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 org.hibernate.validator.constraints.NotEmpty;
+
+import javax.validation.Valid;
+import javax.validation.constraints.NotNull;
+import java.util.List;
+
+public class PreKeyStateV2 {
+
+  @JsonProperty
+  @NotNull
+  @Valid
+  private List<PreKeyV2> preKeys;
+
+  @JsonProperty
+  @NotNull
+  @Valid
+  private DeviceKey deviceKey;
+
+  @JsonProperty
+  @NotNull
+  @Valid
+  private PreKeyV2 lastResortKey;
+
+  @JsonProperty
+  @NotEmpty
+  private String identityKey;
+
+  public PreKeyStateV2() {}
+
+  @VisibleForTesting
+  public PreKeyStateV2(String identityKey, DeviceKey deviceKey, List<PreKeyV2> keys, PreKeyV2 lastResortKey) {
+    this.identityKey   = identityKey;
+    this.deviceKey     = deviceKey;
+    this.preKeys       = keys;
+    this.lastResortKey = lastResortKey;
+  }
+
+  public List<PreKeyV2> getPreKeys() {
+    return preKeys;
+  }
+
+  public DeviceKey getDeviceKey() {
+    return deviceKey;
+  }
+
+  public String getIdentityKey() {
+    return identityKey;
+  }
+
+  public PreKeyV2 getLastResortKey() {
+    return lastResortKey;
+  }
+}