check needed to avoid NPE in HAPI code

* Unique constraints implemented as constraint trigger run after insert
* Constraint trigger functions use postgres advisory transaction locks
to ensure uniqueness checks are not executed in parallel
* Transaction isolation level of insert/update operations changed from
repeatable read to read committed, enabling dirty reads needed to allow
constraint triggers to see inserts/updates executed by parallel running
transactions
* New integration test to validate parallel create operations via
transaction and batch bundles as well as direct POSTs

This does not change any behavior as currently no delete authorization
rule is dependent on the DB state.

log message improved by early wrapping of PSQLException

contains operators @> and <@ only work in one direction, added missing
directions to complete intersection checks