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/PreKeyV2.java

@@ -0,0 +1,82 @@
+package org.whispersystems.textsecuregcm.entities;
+
+/**
+ * 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/>.
+ */
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import org.hibernate.validator.constraints.NotEmpty;
+
+import javax.validation.constraints.NotNull;
+
+public class PreKeyV2 implements PreKeyBase {
+
+  @JsonProperty
+  @NotNull
+  private long    keyId;
+
+  @JsonProperty
+  @NotEmpty
+  private String  publicKey;
+
+  public PreKeyV2() {}
+
+  public PreKeyV2(long keyId, String publicKey)
+  {
+    this.keyId     = keyId;
+    this.publicKey = publicKey;
+  }
+
+  @Override
+  public String getPublicKey() {
+    return publicKey;
+  }
+
+  public void setPublicKey(String publicKey) {
+    this.publicKey = publicKey;
+  }
+
+  @Override
+  public long getKeyId() {
+    return keyId;
+  }
+
+  public void setKeyId(long keyId) {
+    this.keyId = keyId;
+  }
+
+  @Override
+  public boolean equals(Object object) {
+    if (object == null || !(object instanceof PreKeyV2)) return false;
+    PreKeyV2 that = (PreKeyV2)object;
+
+    if (publicKey == null) {
+      return this.keyId == that.keyId && that.publicKey == null;
+    } else {
+      return this.keyId == that.keyId && this.publicKey.equals(that.publicKey);
+    }
+  }
+
+  @Override
+  public int hashCode() {
+    if (publicKey == null) {
+      return (int)this.keyId;
+    } else {
+      return ((int)this.keyId) ^ publicKey.hashCode();
+    }
+  }
+
+}