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/PreKeyCount.java

@@ -0,0 +1,20 @@
+package org.whispersystems.textsecuregcm.entities;
+
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+public class PreKeyCount {
+
+  @JsonProperty
+  private int count;
+
+  public PreKeyCount(int count) {
+    this.count = count;
+  }
+
+  public PreKeyCount() {}
+
+  public int getCount() {
+    return count;
+  }
+}