Handle ix_addk rcode correctly during master swing - #6132

ERR_NOMASTER is treated as a duplicate key error, which causes
upsert to misbehave during master swing: an upsert may miss its
index entry, or may fail incorrectly during verify-retry.

Case 1) upsert misses its index entry

```
mydb4> create table t2 (i int primary key)$$
[create table t2 (i int primary key)] rc 0
```

Upsert receives a good rcode, but misses its index entry:
```
mydb4> insert into t2 values(3) on conflict do nothing
(rows inserted=1)
[insert into t2 values(3) on conflict do nothing] rc 0
mydb4> select * from t2
(i=3)

mydb4> select count(*) from t2 where i = 3
(count(*)=0)
```

Case 2) upsert fails incorrectly during verify-retry

Insert-on-conflict-do-nothing, still receives a dupkey error:

```
mydb4> begin
[begin] rc 0
mydb4> insert into t2 values(6) on conflict do nothing
[insert into t2 values(6) on conflict do nothing] rc 0
mydb4> insert into t2 values(6) on conflict do nothing
[insert into t2 values(6) on conflict do nothing] rc 0
mydb4> update t2 set i = sleep(1) where i = 1
[update t2 set i = sleep(1) where i = 1] rc 0
mydb4> commit
[commit] failed with rc 2 Transaction is uncommittable: Duplicate insert on key 'COMDB2_PK' in table 't2' index 0
```

Database logs the following:

```
[ERROR] Forced VERIFY-FAIL for uncommittable blocksql transaction
```

Signed-off-by: Rivers Zhang <hzhang320@bloomberg.net>