Support for SERIALIZABLE READ ONLY DEFERRABLE?

[bitc]

PostgreSQL supports an additional "isolation level" called SERIALIZABLE READ ONLY DEFERRABLE:

https://www.postgresql.org/docs/13/transaction-iso.html#XACT-SERIALIZABLE

This is a read only transaction in SERIALIZABLE isolation level, but is guaranteed never to retry. It is useful for running long queries (reports).

Thank you

[nikita-volkov]

PR is welcome! Otherwise I don't think I'll get to that update any time soon.