Add CHECK_FOR_INTERRUPTS call when processing sequence nextval().
Add CHECK_FOR_INTERRUPTS call when processing sequence nextval().

After the sequence refactor in eae1ee3fbead8baad3803c64de8cbee07d076d9e,
an INSERT query on a table with a sequence column is not cancellable in
some cases. Specifically, if the QE makes a nextval request, the QD
should process any interrupts, otherwise the QD may continuously receive
requests and never check for interrupts while it's in
checkDispatchResult(). There was no measurable overhead difference with
adding this check.

A repro of this:
```
create table test_kill
(seq_id bigserial
, a bigint
, b text
, c timestamp)
distributed by (seq_id);

 insert into test_kill (a,b,c)
 select id, 'test ' || Id, now()
 from generate_series(1,100000000) Id;
 ```

 After about 2 minutes, attempt to cancel the query on the coordinator
 (once the QD has begun processing sequence values).

xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb