speedup ALTER TABLE ADD CHECK CONSTRAINT.

hi.
attached patch trying to speedup ALTER TABLE ADD CHECK CONSTRAINT.
demo:

drop table if exists t7;
create table t7 (a int not null, b int, c int);
insert into t7 select g, g+1, g %100 from generate_series(1,1_000_000) g;
create index on t7(a,b);

alter table t7 add check (a > 0);
patch: Time: 4.281 ms
master: Time: 491.689 ms
----------------------------
implementation:

step1. during execute command `ALTER TABLE ADD CHECK CONSTRAINT`
in AddRelationNewConstraints->StoreRelCheck
after StoreRelCheck add a CommandCounterIncrement();
so previously added CHECK pg_constraint can be seen by us.
we need to use pg_get_constraintdef to retrieve the CHECK constraint definition.

step2. check if this relation has any suitable index (not expression
index, not predicate index)
--whole patch hinges on SPI query can use indexscan to quickly
retrieve certain information.

step3: construct and execute these three SPI query:
("set enable_seqscan to off")
(SELECT 1 FROM the_table WHERE NOT (check_constraint_def)
AND check_constraint_def IS NOT NULL LIMIT 1)
("reset enable_seqscan")

the SPI query will be faster, if the qual(check_constraint_def) can be
utilized by indexscan.
if SPI_processed == 0 means we toggle this check constraint as
"already_validated" (bool) is true.
we stored already_validated in CookedConstraint. later pass it to
AlteredTableInfo->constraints.
then phrase 3 within ATRewriteTable, we can do
```
if (constraint->already_validated)
needscan = false;
```

----------------------------
concern:
only certain kinds of check constraint expressions can be used for
this optimization.
i do all the CHECK constraint expressions filter in checkconstraint_expr_walker.

use contain_volatile_functions to filter out volatile expressions,
add (check_constraint_def IS NOT NULL) in the above SPI query, i think
null handling is correct?

ALTER TABLE ADD CHECK CONSTRAINT is using ACCESS EXCLUSIVE lock.
but i need to think more about concurrently related issues.

idea come from this thread:
https://postgr.es/m/CANWftzK2MZ7Js_56V+ZcLxZyH1utBZx4uEg03P7Cee86K2roCQ@mail.gmail.com