Make WebSocket optional, disabled by default. Add tests.

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

@@ -1,6 +1,7 @@
 package org.whispersystems.textsecuregcm.entities;
 
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.annotations.VisibleForTesting;
 
 import java.util.List;
 
@@ -12,6 +13,9 @@ public class MismatchedDevices {
   @JsonProperty
   public List<Long> extraDevices;
 
+  @VisibleForTesting
+  public MismatchedDevices() {}
+
   public MismatchedDevices(List<Long> missingDevices, List<Long> extraDevices) {
     this.missingDevices = missingDevices;
     this.extraDevices   = extraDevices;