Allow REPEATABLE READ isolation level.
Allow REPEATABLE READ isolation level.

Forbidding REPEATABLE READ, but allowing SERIALIZABLE, never made much
sense to me. The tragedy in the current situation is that people might
write their applications using SERIALIZABLE, even if plain REPEATABLE READ
is sufficient. They're the same at the moment, but starting with version
9.1, PostgreSQL has a "true" serializable mode that's different, and much
slower, than repeatable read. We don't want applications to use the true
serializable mode in the future, just because we decided to forbid
REPEATABLE READ.

This was discussed on the gpdb-dev mailing list back in 2015, but it didn't
lead to any action then.

Discussion: https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/_gk7LC4rCE8/jaW84dppCAAJ

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