Raise edit level of all accounts before loading transactions and splits

This prevents calling xaccAccountRecomputeBalanceInCurrency on each split that gets added,
which was exponentially increasing load times. On a huge test book the
load time dropped from 53 minutes to 1m20s.

libgnucash/backend/sql/gnc-transaction-sql.cpp

@@ -714,7 +714,13 @@ void
 GncSqlTransBackend::load_all (GncSqlBackend* sql_be)
 {
     g_return_if_fail (sql_be != NULL);
+
+    auto root = gnc_book_get_root_account (sql_be->book());;
+    gnc_account_foreach_descendant(root, (AccountCb)xaccAccountBeginEdit,
+                                   nullptr);
     query_transactions (sql_be, "");
+    gnc_account_foreach_descendant(root, (AccountCb)xaccAccountCommitEdit,
+                                   nullptr);
 }
 
 static void