sql: vectorized inbound streams should handle the first message better

[andreimatei]

The vectorizedInboundStreamHandler ignores the first message from the stream, with no explanation.

cockroach/pkg/sql/colflow/vectorized_flow.go

Line 678 in d3893a1

_ execinfrapb.ProducerMessage,

I think that's OK because the vectorized outboxes don't put any info in that first message besides the stream id, which is used above this level. And I think we don't intermix vectorized and non-vectorized flows on different nodes.

However, this is against the DistSQL protocol; there's nothing mandating that there's no data in that message (in fact, in the row-based outbox case, there is). It's also not clear to me why we can't mix row-based and vectorized flows.

I think we should handle that first message in the vectorized case, or at the very least document and assert that there's no data in that first message.

Jira issue: CRDB-5447

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