pgsql: Fix a gradual memory leak in ExecReScanAgg().

Log Message:
-----------
Fix a gradual memory leak in ExecReScanAgg(). Because the aggregation
hash table is allocated in a child context of the agg node's memory
context, MemoryContextReset() will reset but *not* delete the child
context. Since ExecReScanAgg() proceeds to build a new hash table
from scratch (in a new sub-context), this results in leaking the
header for the previous memory context. Therefore, use
MemoryContextResetAndDeleteChildren() instead.

Credit: My colleague Sailesh Krishnamurthy at Truviso for isolating
the cause of the leak.

Modified Files:
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    pgsql/src/backend/executor:
        nodeAgg.c (r1.152 -> r1.153)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeAgg.c.diff?r1=1.152&r2=1.153)