64-bit CommandIds

Hi,

attached is our patch against HEAD which enables extending CommandIds
to 64-bit. This is for enabling long transactions that really do that much
non-read-only work in one transaction.

The feature is off by default, you need to --enable-huge-commandid.
It fails only one regression test (without_oid) that measures the saved
space in 8.3.

Also, modifying FirstCommandId to be (1<<32ULL - 4) to early overflow
the 32-bit limit) doesn't show any real problem besides the combocid
regression failure that explicitly lists cmin/cmax values, which is
expected.

It was written by Zoltán Böszörményi <zb(at)cybertec(dot)at> and
Hans-Jürgen Schönig <hs(at)cybertec(dot)at>

Best regards,
Zoltán Böszörményi

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Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/