API: Key-based operations

[edrdo]

Intended functionality - example sketch:

    DB db = ... ;
    Table t = table("MyTable")
                  .columns("k1", "k2", "c1", "c2", ...)
                  .keys("k1","k2") // specify columns to consider as primary keys
                  .build(db);
     DataSet data = data(t).row(vk1, vk2, vc1, vc2, ...)
                                        .row( ....)
     ...
     delete(data);     or     update(data);    // key-based operations