Make references as inmemory singletons

[shirokovnv]

Description

Every call of

StaticReferencesData::cadastralDistricts();
StaticReferencesData::subjectCodes();
StaticReferencesData::vehicleFineArticles();
StaticReferencesData::vehicleRegistrationActions();
StaticReferencesData::vehicleRepairMethods();
StaticReferencesData::vehicleCategories();
StaticReferencesData::vehicleTypes();

produces filesystem I/O. Consider storing static data in memory after first open.

Checklist

• [ x] My code follows the style guidelines of this project
• [ x] I have performed a self-review of my own code
• [ x] I have commented my code, particularly in hard-to-understand areas
• I wrote unit tests for my code (if tests is required for my changes)
• [ x] I have made changes in CHANGELOG.md file

Added

• Store references data in memory after first open