Add server time to remote config fetch

Enable clients to very roughly adjust some actions for clock skew by
providing current server time in the remote config fetch.

service/src/main/java/org/whispersystems/textsecuregcm/controllers/RemoteConfigController.java

@@ -15,6 +15,7 @@
 import java.nio.charset.StandardCharsets;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
+import java.time.Clock;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -91,7 +92,7 @@ public UserRemoteConfigList getAll(@Auth AuthenticatedAccount auth) {
             config.getUuids());
         return new UserRemoteConfig(config.getName(), inBucket,
             inBucket ? config.getValue() : config.getDefaultValue());
-      }), globalConfigStream).collect(Collectors.toList()));
+      }), globalConfigStream).collect(Collectors.toList()), Clock.systemUTC().instant());
     } catch (NoSuchAlgorithmException e) {
       throw new AssertionError(e);
     }

service/src/main/java/org/whispersystems/textsecuregcm/entities/UserRemoteConfigList.java

@@ -5,22 +5,33 @@
 
 package org.whispersystems.textsecuregcm.entities;
 
+import com.fasterxml.jackson.annotation.JsonFormat;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
+import java.time.Instant;
 import java.util.List;
 
 public class UserRemoteConfigList {
 
   @JsonProperty
   private List<UserRemoteConfig> config;
 
+  @JsonProperty
+  @JsonFormat(shape = JsonFormat.Shape.NUMBER_INT)
+  private Instant serverEpochTime;
+
   public UserRemoteConfigList() {}
 
-  public UserRemoteConfigList(List<UserRemoteConfig> config) {
+  public UserRemoteConfigList(List<UserRemoteConfig> config, Instant serverEpochTime) {
     this.config = config;
+    this.serverEpochTime = serverEpochTime;
   }
 
   public List<UserRemoteConfig> getConfig() {
     return config;
   }
+
+  public Instant getServerEpochTime() {
+    return serverEpochTime;
+  }
 }