Improve manifest relaying and domain reporting:

- The changes to manifest relaying introduced with commit f74b469e68
  will cause newly accepted manifests to be sent back to the peer from
  which they were received. This no longer happens: a newly accepted
  manifest is never sent back to the peer we received it from.
- When encountering a manifest without a domain set, the `manifest` and
  `validator_info` commands would include an empty string as the domain
  associated with the manifest. This no longer happens: if a domain is
  not present, the `domain` field will not be.

src/test/rpc/ValidatorInfo_test.cpp

@@ -106,7 +106,6 @@ public:
             BEAST_EXPECT(info[jss::result][jss::manifest] == manifest);
             BEAST_EXPECT(
                 info[jss::result][jss::ephemeral_key] == ephemeral_key);
-            BEAST_EXPECT(info[jss::result][jss::domain] == "");
         }
     }