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

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.