Add table AM 'tid_visible'

We directly call the heap function VM_ALL_VISIBLE in the

IndexOnlyNext function. This is not in line with the design idea of

table am. If the new storage type needs to implement index only

scan, he must hack the IndexOnlyNext function.

So this patch add a new table am 'tid_visible' to test visibility

of tid. So that index only scan can completely use table AM.