pgsql: Use type "int64" for memory accounting in tuplesort.c/tuplestore

Use type "int64" for memory accounting in tuplesort.c/tuplestore.c.

Commit 263865a48973767ce8ed7b7788059a38a24a9f37 switched tuplesort.c and
tuplestore.c variables representing memory usage from type "long" to
type "Size". This was unnecessary; I thought doing so avoided overflow
scenarios on 64-bit Windows, but guc.c already limited work_mem so as to
prevent the overflow. It was also incomplete, not touching the logic
that assumed a signed data type. Change the affected variables to
"int64". This is perfect for 64-bit platforms, and it reduces the need
to contemplate platform-specific overflow scenarios. It also puts us
close to being able to support work_mem over 2 GiB on 64-bit Windows.

Per report from Andres Freund.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/79e0f87a15643efa9a94e011da509746dbb96798

Modified Files
--------------
src/backend/utils/sort/tuplesort.c | 30 ++++++++++++++++--------------
src/backend/utils/sort/tuplestore.c | 14 ++++++++------
src/include/utils/tuplesort.h | 2 +-
3 files changed, 25 insertions(+), 21 deletions(-)