Adapt DbTable to use only 'new style' for extra functions, thereby remove the extend.

Signed-off-by: Norv <a.w.norv@gmail.com>
Refactor the $reservedTables global into an instance var. It's only used in DbTable.
(could be protected too)

Signed-off-by: Norv <a.w.norv@gmail.com>
Document our new and shiny var.

Signed-off-by: Norv <a.w.norv@gmail.com>