Rearrange provisioning flow. Add needsMessageSync response.

// FREEBIE

src/main/java/org/whispersystems/textsecuregcm/entities/ProvisioningMessage.java

@@ -1,18 +1,14 @@
 package org.whispersystems.textsecuregcm.entities;
 
 import com.fasterxml.jackson.annotation.JsonProperty;
+import org.hibernate.validator.constraints.NotEmpty;
 
 public class ProvisioningMessage {
 
   @JsonProperty
+  @NotEmpty
   private String body;
 
-  public ProvisioningMessage() {}
-
-  public ProvisioningMessage(String body) {
-    this.body = body;
-  }
-
   public String getBody() {
     return body;
   }

src/main/java/org/whispersystems/textsecuregcm/entities/SendMessageResponse.java

@@ -0,0 +1,16 @@
+package org.whispersystems.textsecuregcm.entities;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+public class SendMessageResponse {
+
+  @JsonProperty
+  private boolean needsSync;
+
+  public SendMessageResponse() {}
+
+  public SendMessageResponse(boolean needsSync) {
+    this.needsSync = needsSync;
+  }
+
+}