Don't take ProcArrayLock while exiting a transaction that has no XID; there is

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

Commit: 0a51e7073c045b5fce50092dae19f398f7b38e16
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2007-09-07T20:59:26Z
Releases: 8.3.0
Don't take ProcArrayLock while exiting a transaction that has no XID; there is
no need for serialization against snapshot-taking because the xact doesn't
affect anyone else's snapshot anyway.  Per discussion.  Also, move various
info about the interlocking of transactions and snapshots out of code comments
and into a hopefully-more-cohesive discussion in access/transam/README.

Also, remove a couple of now-obsolete comments about having to force some WAL
to be written to persuade RecordTransactionCommit to do its thing.

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