Add an "excessively large object" safeguard.

service/src/main/java/org/whispersystems/textsecuregcm/configuration/TorExitNodeConfiguration.java

@@ -25,6 +25,9 @@ public class TorExitNodeConfiguration {
   @NotBlank
   private String objectKey;
 
+  @JsonProperty
+  private long maxSize = 16 * 1024 * 1024;
+
   @JsonProperty
   private Duration refreshInterval = Duration.ofMinutes(5);
 
@@ -45,6 +48,15 @@ public class TorExitNodeConfiguration {
     return objectKey;
   }
 
+  public long getMaxSize() {
+    return maxSize;
+  }
+
+  @VisibleForTesting
+  public void setMaxSize(final long maxSize) {
+    this.maxSize = maxSize;
+  }
+
   public Duration getRefreshInterval() {
     return refreshInterval;
   }