Aligned structure of connector traits with other traits
Streamlined type traits a bit
Cleaned up traits a bit more
Merge branch 'release/0.18'
Merge tag '0.18' into develop

* Streamlined type traits.
* Transport can_be_null to result fields
* Connector trait enforce_null_result_treatment prohibits direct conversion of can_be_null-fields to cpp_value_type
* Column spec trait null_is_trivial_value overrides