[YSQL] Support DEFERRABLE constraints

[m-iancu]

Jira Link: DB-2213
This is a master issue that keeps track of all deferrable constraints variants.
The check mark means that the statement is already supported.

Feature	Status	Tracking Issue	Comments
Foreign Key DEFERRABLE INITIALLY IMMEDIATE	✅	#3995
Foreign Key DEFERRRABLE INITIALLY DEFERRED	✅	#3995
Unique/Primary key DEFERRABLE INITIALLY IMMEDIATE	⬜️	#1709
Unique/Primary key DEFERRABLE INITIALLY DEFERRED	⬜️	#1709
Constraint triggers	⬜️		Constraint triggers not yet supported.
Exclusion constraint	⬜️		Exclusion constraints not yet supported.

Dependent ecosystem projects:

Project	Status	Issues
Django	⬜️	#4054

[bllewell]

The graphic above claims that deferred FK constraints work. Issue #13009 shows that they don't work in one of the examples that text books and bloggers frequently use to illustrate why deferred FK constraints are essential:

the optional (or mandatory) one-to-one relationship

Moreover, especially for the case that the PKs of the tables are automatically generated, proper practice recommends a stored procedure encapsulation of the three SQLs (insert returning, insert returning, and update using the returned values) that must be done in a single transaction.

As of this comment's timestamp, example code for this use case works faultlessly in vanilla PG. But it fails in YB:

• In the pure SQL implementation for the mandatory 1:1 case, you get a nonsensical error message that fails the txn.
• In the PG/pgSQL implementation for both the optional and the mandatory 1:1 case, you get a crash and the connection to the server is lost.