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/core/impl/SociDB.cpp

@@ -160,6 +160,16 @@ void convert (std::vector<std::uint8_t> const& from, soci::blob& to)
 {
     if (!from.empty ())
         to.write (0, reinterpret_cast<char const*>(&from[0]), from.size ());
+    else
+        to.trim (0);
+}
+
+void convert (std::string const& from, soci::blob& to)
+{
+    if (!from.empty ())
+        to.write (0, from.data (), from.size ());
+    else
+        to.trim (0);
 }
 
 namespace {