Fix amendment voting persistence:

An incorrect SQL query could cause the server to improperly
configure its voting state after a restart; typically, this
would manifest as an apparent failure to store a vote which
the administrator of the server had configured.

This commit fixes the broken SQL and ensures that amendment
votes are properly reloaded post-restart and closes #4220.

src/ripple/app/rdb/impl/Wallet.cpp

@@ -254,7 +254,10 @@ readAmendments(
 
     soci::transaction tr(session);
     std::string sql =
-        "SELECT AmendmentHash, AmendmentName, Veto FROM FeatureVotes";
+        "SELECT AmendmentHash, AmendmentName, Veto FROM "
+        "( SELECT AmendmentHash, AmendmentName, Veto, RANK() OVER "
+        "(  PARTITION BY AmendmentHash ORDER BY ROWID DESC ) "
+        "as rnk FROM FeatureVotes ) WHERE rnk = 1";
     // SOCI requires boost::optional (not std::optional) as parameters.
     boost::optional<std::string> amendment_hash;
     boost::optional<std::string> amendment_name;