sql: columnar direct scan results in 10x more messages

Setup:
```
create table t(i int, s string);
insert into t select generate_series(1,100000), repeat("asdfasdf", 1024);
```
with columnar direct scan:

```
maximum memory usage: 144 KiB
network usage: 7.2 MiB (7,261 messages)
sql cpu time: 4ms
```
w/o
```
maximum memory usage: 10 MiB
network usage: 6.4 MiB (805 messages)
sql cpu time: 95ms
```

Jira issue: CRDB-26103