Rearrange pg_subtrans handling as per recent discussion. pg_subtrans

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 4dbb880d3c77a580ef9634b93af26eaf64484ddb
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2004-08-23T23:22:45Z
Releases: 8.0.0
Rearrange pg_subtrans handling as per recent discussion.  pg_subtrans
updates are no longer WAL-logged nor even fsync'd; we do not need to,
since after a crash no old pg_subtrans data is needed again.  We truncate
pg_subtrans to RecentGlobalXmin at each checkpoint.  slru.c's API is
refactored a little bit to separate out the necessary decisions.

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