pgsql: Pull up isReset flag from AllocSetContext to MemoryContext struc

Pull up isReset flag from AllocSetContext to MemoryContext struct. This
avoids the overhead of one function call when calling MemoryContextReset(),
and it seems like the isReset optimization would be applicable to any new
memory context we might invent in the future anyway.

This buys back the overhead I just added in previous patch to always call
MemoryContextReset() in ExecScan, even when there's no quals or projections.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/30e98a7e6e4869a7d6b3748ac9770bb8d69a8b26

Modified Files
--------------
src/backend/utils/mmgr/aset.c |   25 ++-----------------------
src/backend/utils/mmgr/mcxt.c |   18 ++++++++++++++++--
src/include/nodes/memnodes.h  |    1 +
3 files changed, 19 insertions(+), 25 deletions(-)