dsm use of uint64

When I wrote the dynamic shared memory patch, I used uint64 everywhere
to measure sizes - rather than, as we do for the main shared memory
segment, Size. This now seems to me to have been the wrong decision;
I'm finding that it's advantageous to make dynamic shared memory
behave as much like the main shared memory segment as is reasonably
possible, and using Size facilitates the use of MAXALIGN(),
TYPEALIGN(), etc. as well as things like add_size() and mul_size()
which are just as relevant in the dynamic shared memory case as they
are for the main shared memory segment.

Therefore, I propose to apply the attached patch.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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