Re-coded for cached locale.

Testing notes:  Based on the averages of 3 runs, the net
user CPU to save the XML file I use is:
10.2 seconds without this change
7.6 seconds with this change

In my environment the first call to the format routine
in question, the call that sets the cache value, is at
the end of the XML load.

libgnucash/engine/gnc-datetime.cpp

@@ -425,11 +425,21 @@ std::string
 GncDateTimeImpl::format(const char* format) const
 {
     using Facet = boost::local_time::local_time_facet;
+    static std::locale cachedLocale;
+    static bool cachedLocaleAvailable = false;
     std::stringstream ss;
+
+    if(!cachedLocaleAvailable)
+    {
+        cachedLocale = std::locale("");
+	cachedLocaleAvailable = true;
+    }
+
     //The stream destructor frees the facet, so it must be heap-allocated.
     auto output_facet(new Facet(normalize_format(format).c_str()));
     // FIXME Rather than imbueing a locale below we probably should set std::locale::global appropriately somewhere.
-    ss.imbue(std::locale(std::locale(""), output_facet));
+    // At that point the use of cachedLocale mechanism should be removed.
+    ss.imbue(std::locale(cachedLocale, output_facet));
     ss << m_time;
     return ss.str();
 }