Tidying & Selectively forward manifests to peers:

* Do not forward manifests to peers that already know that manifest
* Do not forward historical manifests to peers
* Save/Load ValidatorManifests from a database
* Python test for setting ephmeral keys
* Cleanup manifest interface

src/ripple/app/main/Application.cpp

@@ -806,6 +806,8 @@ public:
             getConfig());
         add (*m_overlay); // add to PropertyStream
 
+        m_overlay->setupValidatorKeyManifests (getConfig (), getWalletDB ());
+
         {
             auto setup = setup_ServerHandler(getConfig(), std::cerr);
             setup.makeContexts();
@@ -900,6 +902,8 @@ public:
 
         mValidations->flush ();
 
+        m_overlay->saveValidatorKeyManifests (getWalletDB ());
+
         RippleAddress::clearCache ();
         stopped ();
     }