Index only scan for cube and seg

Hi hackers!

Here are patches enabling Index Only Scan for cube and seg extensions.

These patches follow this discussion [0].

For cube there is new default opclass. We cannot drop old opclass, because it could TOAST come cube values in rare occasions. Index Only Scan is enabled only for newly created indexes. Btw I can add fetch to old opclass so that IOS would be enabled.
For seg compress and decompress functions are dropped from opclass and extension. Index Only Scan is enabled.

There are two more functions which can be deleted
ghstore_decompress
g_intbig_decompress
But it will not lead to any feasible consequences.


[0] https://www.postgresql.org/message-id/flat/CAJEAwVELVx9gYscpE%3DBe6iJxvdW5unZ_LkcAaVNSeOwvdwtD%3DA%40mail.gmail.com#CAJEAwVELVx9gYscpE=Be6iJxvdW5unZ_LkcAaVNSeOwvdwtD=A@mail.gmail.com