fix: increment manifest sequence for client code cache invalidation

OverlayImpl::getManifestsMessage() caches manifest messages and only rebuilds
them when ManifestCache::sequence() changes. When accepting a new manifest,
the function returned early without incrementing seq_, causing the cache to
never invalidate. This meant peers were exchanging stale lists.

Now seq_++ is called for both new manifests and updates, ensuring the overlay
layer detects changes and sends complete validator lists to connecting peers.

Closes #629

src/ripple/app/misc/impl/Manifest.cpp

@@ -471,6 +471,10 @@ ManifestCache::applyManifest(Manifest m)
 
         auto masterKey = m.masterKey;
         map_.emplace(std::move(masterKey), std::move(m));
+
+        // Increment sequence to invalidate cached manifest messages
+        seq_++;
+
         return ManifestDisposition::accepted;
     }