Add callback for I/O error messages in SLRUs

Add callback for I/O error messages in SLRUs

Historically, all SLRUs were addressed by transaction IDs, but that
hasn't been true for a long time. However, the error message on I/O
error still always talked about accessing a transaction ID.

This commit adds a callback that allows subsystems to construct their
own error messages, which can then correctly refer to a transaction
ID, multixid or whatever else is used to address the particular SLRU.

Author: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://www.postgresql.org/message-id/CACG=ezZZfurhYV+66ceubxQAyWqv9vaUi0yoO4-t48OE5xc0DQ@mail.gmail.com