Update mutant dependencies
Add support for MemoryScope#where with no arguments
Add support for MemoryScope#not
Add setup for database_cleaner in core

* Cleanup test DB prior each suite run for reproducible results
Add optional database cleaning transaction around examples
Change CreateItemAdjustmentsSpec to be executed isolated
Fix style to ease next changes
Kill relevant mutations in CreateItemAdjustments

* There is one semantically not relevant mutation left that fixes a
  performance problem.
Use in memory state to create adjustments

* Reduces transitive lazy loads.
* Refactors away last alive mutation without introducing a performance
  regression.