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/core/impl/LoadMonitor.cpp

@@ -78,7 +78,6 @@ LoadMonitor::update()
         mLatencyMSAvg = 0ms;
         mLatencyMSPeak = 0ms;
         mLastUpdate = now;
-        // VFALCO TODO don't return from the middle...
         return;
     }