pgsql: Improve shm_mq portability around MAXIMUM_ALIGNOF and sizeof(Siz

Improve shm_mq portability around MAXIMUM_ALIGNOF and sizeof(Size).

Revise the original decision to expose a uint64-based interface and
use Size everywhere possible. Avoid assuming that MAXIMUM_ALIGNOF is
8, or making any assumption about the relationship between that value
and sizeof(Size). If MAXIMUM_ALIGNOF is bigger, we'll now insert
padding after the length word; if it's smaller, we are now prepared
to read and write the length word in chunks.

Per discussion with Tom Lane.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3bd261ca18c67eafe18088e58fab511e3b965418

Modified Files
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src/backend/storage/ipc/shm_mq.c | 251 +++++++++++++++++++++++---------------
src/include/storage/shm_mq.h | 4 +-
2 files changed, 154 insertions(+), 101 deletions(-)