Demo for update-only table

[bobvawter]

This is an upsert template hack to make a table update-only. This could also be accomplished though a custom apply function, which would make a nice demo of either diff options or by using a SQL call to ping the table row. We could also choose to support an insert- or update-only mode for some table as a baked-in behavior.

{{- /*gotype: github.com/cockroachdb/replicator/internal/target/apply.templates*/ -}}
{{- /*
This is a demonstration template to only perform updates over existing rows.
*/ -}}
WITH raw_data( {{- template "names" .Columns -}} ) AS (
VALUES{{- nl -}}
{{- template "exprs" . -}}
)
UPDATE {{ .TableName }} t SET {{- nl -}}
{{- range $idx, $col := .Data -}}
    {{- if $idx -}},{{- end -}}
    {{- $col.Name -}}=d.{{- $col.Name -}}
{{- end -}} {{- nl -}}
FROM raw_data d
WHERE {{- nl -}}
{{- range $idx, $col := .PK -}}
    {{- if $idx }} AND {{ end -}}
    t.{{- $col.Name -}}=d.{{- $col.Name -}}
{{- end -}}
{{- /* Trim whitespace */ -}}