More comfortable serialization failure catching for PostgreSQL

[tuxzz]

As the PostgreSQL document said, when using transaction of isolation level Repeatable read or Serializable, database may return error called Serialization Failure (Error code 40001), which is a very, very common error. Applications using these level must be prepared to retry transactions due to serialization failures.

Maybe we can implement something like auto-retry for PostgreSQL after #481 is solved?