Remove disappearing family members.

When a relay stops writing any family members into its server
descriptor we never cleared old family members but kept them forever.
We should do that.

Fixes #28314.

CHANGELOG.md

+# Changes in version 7.0-1.2?.? - 2019-1?-??
+
+ * Medium changes
+   - Remove declared/alleged family members after they disappear from
+     server descriptors.
+
+
 # Changes in version 7.0-1.21.1 - 2019-11-09
 
  * Minor changes

src/main/java/org/torproject/metrics/onionoo/updater/NodeDetailsStatusUpdater.java

@@ -172,8 +172,8 @@ public class NodeDetailsStatusUpdater implements DescriptorListener,
     detailsStatus.setExitPolicy(descriptor.getExitPolicyLines());
     detailsStatus.setContact(descriptor.getContact());
     detailsStatus.setPlatform(descriptor.getPlatform());
+    SortedSet<String> declaredFamily = new TreeSet<>();
     if (descriptor.getFamilyEntries() != null) {
-      SortedSet<String> declaredFamily = new TreeSet<>();
       for (String familyMember : descriptor.getFamilyEntries()) {
         if (familyMember.startsWith("$") && familyMember.length() >= 41) {
           declaredFamily.add(
@@ -182,8 +182,8 @@ public class NodeDetailsStatusUpdater implements DescriptorListener,
           declaredFamily.add(familyMember);
         }
       }
-      this.declaredFamilies.put(fingerprint, declaredFamily);
     }
+    this.declaredFamilies.put(fingerprint, declaredFamily);
     if (descriptor.getIpv6DefaultPolicy() != null
         && (descriptor.getIpv6DefaultPolicy().equals("accept")
         || descriptor.getIpv6DefaultPolicy().equals("reject"))