cubicweb #2410822 Sql store [done]

Add a store based on direct SQL insertions, using COPY FROM.

prioritynormal
typeenhancement
done in3.16.0
load0.500
load left0.000
closed by#7ee0752178e5 [dataimport] Add SQL Store for faster import - works ONLY with Postgres for now, as it requires "copy from" command - closes #2410822