9.5: Better memory accounting, towards memory-bounded HashAgg

Attached is a patch that explicitly tracks allocated memory (the blocks,
not the chunks) for each memory context, as well as its children.

This is a prerequisite for memory-bounded HashAgg, which I intend to
submit for the next CF. Hashjoin tracks the tuple sizes that it adds to
the hash table, which is a good estimate for Hashjoin. But I don't think
it's as easy for Hashagg, for which we need to track transition values,
etc. (also, for HashAgg, I expect that the overhead will be more
significant than for Hashjoin). If we track the space used by the memory
contexts directly, it's easier and more accurate.

I did some simple pgbench select-only tests, and I didn't see any TPS
difference.

Regards,
Jeff Davis