Return Retry-After time to clients when they are rate limited (#421)

* Return Retry-After time to clients when they are rate limited

* Update based on feedback

- New exception type that is mapped differently
- Always report time until allowed on rate limits
- Consume and transform into a differnt exception if we think it will be
  allowed later

service/src/main/java/org/whispersystems/textsecuregcm/mappers/RetryLaterExceptionMapper.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.mappers;
+
+import org.whispersystems.textsecuregcm.controllers.RateLimitExceededException;
+import javax.ws.rs.core.Response;
+import javax.ws.rs.ext.ExceptionMapper;
+import javax.ws.rs.ext.Provider;
+
+import org.whispersystems.textsecuregcm.controllers.RetryLaterException;
+
+import javax.ws.rs.core.Response;
+import javax.ws.rs.ext.ExceptionMapper;
+import javax.ws.rs.ext.Provider;
+import java.time.Duration;
+
+@Provider
+public class RetryLaterExceptionMapper implements ExceptionMapper<RetryLaterException> {
+  @Override
+  public Response toResponse(RetryLaterException e) {
+    return Response.status(413)
+                   .header("Retry-After", e.getBackoffDuration().toSeconds())
+                   .build();
+  }
+}
+