cubicweb #1794006 use names PK constraints [open]

The PK constraints for entities are not named explicitely (and use default autogenreated names by the backend). Fixing this would enable easier administration and cross db backend support.

Yams already uses explicit names for relation tables.

CREATE TABLE ..... CONSTRAINT index_name PRIMARY KEY (col1, col2...) CREATE TABLE tablename ( colname coltype CONSTRAINT indexname PRIMARY KEY, ...)

done in<not specified>
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