Bug 791848 - GC 2.6.x does not handle ISO dates introduced with GC 2.7.

Set a feature to prevent versions older that 2.6.20 from loading a
database from which they cannot read the dates. Ideally we would do this
only if the database is written to, but the current persistence design
includes committing back to the database during the load so the net
effect is that the flag would be set anyway.

libgnucash/backend/dbi/gnc-backend-dbi.cpp

@@ -59,7 +59,7 @@ extern "C"
 #include "gnc-engine.h"
 #include "SX-book.h"
 #include "Recurrence.h"
-
+#include <gnc-features.h>
 #include "gnc-uri-utils.h"
 #include "gnc-filepath-utils.h"
 #include <gnc-path.h>
@@ -76,6 +76,7 @@ extern "C"
 #include <string>
 #include <iomanip>
 
+#include <qofsession.hpp>
 #include <gnc-backend-prov.hpp>
 #include "gnc-backend-dbi.h"
 #include "gnc-backend-dbi.hpp"
@@ -470,7 +471,6 @@ GncDbiBackend<DbType::DBI_SQLITE>::session_begin(QofSession* session,
      * Let's start logging */
     xaccLogSetBaseName (filepath.c_str());
     PINFO ("logpath=%s", filepath.c_str() ? filepath.c_str() : "(null)");
-
     LEAVE ("");
 }
 
@@ -830,6 +830,9 @@ GncDbiBackend<Type>::load (QofBook* book, QofBackendLoadType loadType)
 
     GncSqlBackend::load(book, loadType);
 
+    if (Type == DbType::DBI_SQLITE)
+        gnc_features_set_used(book, GNC_FEATURE_SQLITE3_ISO_DATES);
+
     if (GNUCASH_RESAVE_VERSION > get_table_version("Gnucash"))
     {
         /* The database was loaded with an older database schema or