Work around a bug in insert_record, where the table columns were cached for a table the first time the table was inserted into, making it impossible to do inserts into the table in the same request if the table is changed.

Added a global variable, $INSERTRECORD_NOCACHE, that can be set in order to force re-looking up the table columns.

Use this feature to fix the installation of the forum interaction activity type "new post" when upgrading.