Relax 1:1 connection constraints (under an option)

[Totktonada]

• Use name 1:1 for all 1:1 and 1:1* connections to simplify setup, but enforce constraints depending on user-provided options.
• Allow to violate FULL MATCH constraint (see Write tests for a nullable index on shard #43) under an option: have both nulls and non-nulls in source fields set of an connection (need to check again three constraint types in Write tests for a nullable index on shard #43, maybe we can provide ability to choose an one of them).
• Allow dandling connections under an option: source fields set of a connection that does not match anything in a destination collection.
• Improve 'dangling connection' detection: remove false positive when filters passed for a connection or remove this constraint completely.

[Totktonada]

Decided to introduce an option to allow dandling 1:1 connections, but don’t introcude an option to violate all-null-or-all-non-null FULL MATCH constraint (no need for now).