Clean up and modernize code:

This commit removes obsolete comments, dead or no longer useful
code, and workarounds for several issues that were present in older
compilers that we no longer support.

Specifically:

- It improves the transaction metadata handling class, simplifying
  its use and making it less error-prone.
- It reduces the footprint of the Serializer class by consolidating
  code and leveraging templates.
- It cleanups the ST* class hierarchy, removing dead code, improving
  and consolidating code to reduce complexity and code duplication.
- It shores up the handling of currency codes and the conversation
  between 160-bit currency codes and their string representation.
- It migrates beast::secure_erase to the ripple namespace and uses
  a call to OpenSSL_cleanse instead of the custom implementation.

src/ripple/ledger/impl/ApplyStateTable.cpp

@@ -123,9 +123,7 @@ ApplyStateTable::apply(
     std::shared_ptr<Serializer> sMeta;
     if (!to.open())
     {
-        TxMeta meta;
-        // VFALCO Shouldn't TxMeta ctor do this?
-        meta.init(tx.getTransactionID(), to.seq());
+        TxMeta meta(tx.getTransactionID(), to.seq());
         if (deliver)
             meta.setDeliveredAmount(*deliver);
         Mods newMod;
@@ -526,12 +524,24 @@ ApplyStateTable::threadItem(TxMeta& meta, std::shared_ptr<SLE> const& sle)
 {
     key_type prevTxID;
     LedgerIndex prevLgrID;
+
     if (!sle->thread(meta.getTxID(), meta.getLgrSeq(), prevTxID, prevLgrID))
         return;
-    if (prevTxID.isZero())
-        return;
-    TxMeta::thread(
-        meta.getAffectedNode(sle, sfModifiedNode), prevTxID, prevLgrID);
+
+    if (!prevTxID.isZero())
+    {
+        auto& node = meta.getAffectedNode(sle, sfModifiedNode);
+
+        if (node.getFieldIndex(sfPreviousTxnID) == -1)
+        {
+            assert(node.getFieldIndex(sfPreviousTxnLgrSeq) == -1);
+            node.setFieldH256(sfPreviousTxnID, prevTxID);
+            node.setFieldU32(sfPreviousTxnLgrSeq, prevLgrID);
+        }
+
+        assert(node.getFieldH256(sfPreviousTxnID) == prevTxID);
+        assert(node.getFieldU32(sfPreviousTxnLgrSeq) == prevLgrID);
+    }
 }
 
 std::shared_ptr<SLE>