This optimization reuses an existing cursor when op_open_write() is
called on the same table/index (same root_page). This is safe because
the cursor position doesn't matter - op_rewind() is always called
after op_open_write() to position the cursor at the beginning of the
table/index before any operations are performed.
This change speeds up op_open_write() by avoiding unnecessary cursor
re-initialization.
413c582b41