pgsql: Fix race condition in multixact code: it's possible to try to

Log Message:
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Fix race condition in multixact code: it's possible to try to read a
multixact's starting offset before the offset has been stored into the
SLRU file.  A simple fix would be to hold the MultiXactGenLock until the
offset has been stored, but that looks like a big concurrency hit.  Instead
rely on knowledge that unset offsets will be zero, and loop when we see
a zero.  This requires a little extra hacking to ensure that zero is never
a valid value for the offset.  Problem reported by Matteo Beccati, fix
ideas from Martijn van Oosterhout, Alvaro Herrera, and Tom Lane.

Modified Files:
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    pgsql/src/backend/access/transam:
        multixact.c (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/multixact.c.diff?r1=1.9&r2=1.10)