Reduce likelihood of malformed SOTemplate:

Formerly an SOTemplate was default constructed and its elements
added using push_back().  This left open the possibility of a
malformed SOTemplate if adding one of the elements caused a throw.

With this commit the SOTemplate requires an initializer_list of
its elements at construction.  Elements may not be added after
construction.  With this approach either the SOTemplate is fully
constructed with all of its elements or the constructor throws,
which prevents an invalid SOTemplate from even existing.

This change requires all SOTemplate construction to be adjusted
at the call site.  Those changes are also in this commit.

The SOE_Flags enum is also renamed to SOEStyle, which harmonizes
the name with other uses in the code base.  SOEStyle elements
are renamed (slightly) to have an "soe" prefix rather than "SOE_".
This heads toward reserving identifiers with all upper case for
macros.  The new style also aligns with other prominent enums in
the code base like the collection of TER identifiers.

SOElement is adjusted so it can be stored directly in an STL
container, rather than requiring storage in a unique_ptr.
Correspondingly, unique_ptr usage is removed from both
SOTemplate and KnownFormats.

src/ripple/protocol/TxFormats.h

@@ -65,14 +65,12 @@ enum TxType
 class TxFormats : public KnownFormats <TxType>
 {
 private:
-    void addCommonFields (Item& item) override;
-
-public:
     /** Create the object.
-        This will load the object will all the known transaction formats.
+        This will load the object with all the known transaction formats.
     */
     TxFormats ();
 
+public:
     static TxFormats const& getInstance ();
 };