Query Engine: Multi field unique

[prisma]

Query Engine: Multi field unique

[mavilein]

We can start this after the asyncification of the query engine is done.

[schickling]

Note: Once this issue is complete, we need to address #68 in order to solve prisma/prisma#773

[dpetrick]

Task Breakdown [WIP]:
Core:

• Schema must include a combination fields as unique criteria as per definition of the constraint.
• Query parsing & validation: Investigate if changes are required.
• Query graph: Investigate if changes are required.
• RecordFinder (and possibly Filter) structs need to change so it can store a combined unique criteria.
• Extensive testsuite required to cover all scenarios where multi field uniques can be used.

Connectors:

• SQL connector implementation needs to change and adapt the multi field unique queries.