pgsql: Move out-of-memory error checks from aset.c to mcxt.c

Move out-of-memory error checks from aset.c to mcxt.c

This potentially allows us to add mcxt.c interfaces that do something
other than throw an error when memory cannot be allocated.  We'll
handle adding those interfaces in a separate commit.

Michael Paquier, with minor changes by me

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3d6d1b585524aab62616d20518fe1fe3be5d1a39

Modified Files
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src/backend/utils/mmgr/aset.c |   39 ++++++++----------------
src/backend/utils/mmgr/mcxt.c |   66 +++++++++++++++++++++++++++++++++++++++++
2 files changed, 79 insertions(+), 26 deletions(-)