delta_append_bat: let comment point to COPY INTO LOCKED as alternative
delta_append_bat: let comment point to COPY INTO LOCKED as alternative

Since changeset 8de58bfc4c1f, delta_append_bat (and thus sql.append())
does no longer re-use a BAT when bulk-loading into an empty table,
but rather requires an additional copy.
This can be avoided by using COPY INTO ... LOCKED.
The performance difference with TPCH SF-100 on a 16 GB machine
is a factor 3.5 .

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