Improve roundtrip verification tests and debug dump.

Add another round-trip verification.  For both original and generated
state, generate a diff from an initial, empty state.  Verify that
these two diffs are the same.

src/statesync/user.h

@@ -89,6 +89,7 @@ namespace Network {
     /* interface for Network::Transport */
     void subtract( const UserStream *prefix );
     string diff_from( const UserStream &existing ) const;
+    string init_diff( void ) const { assert( false ); return string(); };
     void apply_string( string diff );
     bool operator==( const UserStream &x ) const { return actions == x.actions; }