epic: subset system revision

[wwoytenko]

Greenmask includes a subset system that helps users consistently reduce database size based on specified conditions. However, there are currently limitations that prevent this system from being used effectively in databases with cyclic references.

High-level tasks to implement:

• Implement a common subset system for all RDBMSs (already introduced in the MySQL adaptation by adding a new subset framework, epic: MySQL support #222)
• Implement virtual foreign key definitions in the new subset framework
• Implement virtual primary key definitions in the subset framework (feat: Virtual primary keys #319, Specify the primary key of the referenced table in a virtual reference #270)
• Port the PostgreSQL subset system from v0.2 to the new rails (v1)
• Research cycle resolution approaches for RDBMSs that support recursive queries
• Implement a new version of the cycle resolver based on recursive queries
• Resolve known bugs and issues (greenmask does not work if there is not cycle in the database #329, bug: index out of range [0] with length 0 in generateOverlapQuery during subset #279, feat: Advanced Subset: Multi-cycles with more than one group in one SCC #197)
• Revise subset documentation for all supported DBMS engines
• Polymorphic condition issue Bug: Subset - polymorphic scope predicates missing OR NOT bypass guard - multi-type refs produce ~0 rows #396
• Issues Subset error #403