[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