[PATCH] Add initial xid/mxid/mxoff to initdb

Hi!

During work on 64-bit XID patch [1] we found handy to have initdb options
to set initial xid/mxid/mxoff values to arbitrary non default values. It
helps test different scenarios: related to wraparound, pg_upgrade from
32-bit XID to 64-bit XID, etc.

We realize, this patch can be singled out as an independent patch from the
whole patchset in [1] and be useful irrespective of 64-bit XID in cases of
testing of wraparound and so on.

In particular, we employed this patch to test recent changes in logical
replication of subxacts [2] and found no problems in it near the point of
publisher wraparound.

Please share your opinions and reviews are always welcome.

[1]
https://www.postgresql.org/message-id/flat/CACG%3DezZe1NQSCnfHOr78AtAZxJZeCvxrts0ygrxYwe%3DpyyjVWA%40mail.gmail.com
[2]
https://postgr.es/m/d045f3c2-6cfb-06d3-5540-e63c320df8bc@enterprisedb.com

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Best regards,
Maxim Orlov.