Improve manifest relaying:

The manifest relay code would only ever relay manifests from validators
on a server's UNL which means that the manifests of validators that are
not broadly trusted can fail to propagate across the network, which can
make it difficult to detect and track such validators.

This commit, if merged, propagates all manifests on a best-effort basis
resulting in broader availability of manifests on the network and avoid
the need to introduce on-ledger manifest storage or to establish one or
more manifest repositories.

src/ripple/app/misc/NetworkOPs.cpp

@@ -1913,6 +1913,8 @@ NetworkOPsImp::pubManifest(Manifest const& mo)
         if (auto sig = mo.getSignature())
             jvObj[jss::signature] = strHex(*sig);
         jvObj[jss::master_signature] = strHex(mo.getMasterSignature());
+        if (!mo.domain.empty())
+            jvObj[jss::domain] = mo.domain;
 
         for (auto i = mStreamMaps[sManifests].begin();
              i != mStreamMaps[sManifests].end();)