We introduce a new type of memory pool and memory pool manager:
CMemoryPoolPalloc and CMemoryPoolPallocManager

The motivation for this PR is to improve memory allocation/deallocation
performance when using GPDB allocators. Additionally, we would like to
use the GPDB memory allocators by default (change the default for
optimizer_use_gpdb_allocators to on), to prevent ORCA from crashing when
we run out of memory (OOM). However, with the current way of doing
things, doing so would add around 10 % performance overhead to ORCA.

CMemoryPoolPallocManager overrides the default CMemoryPoolManager in
ORCA, and instead creates a CMemoryPoolPalloc memory pool instead of a
CMemoryPoolTracker. In CMemoryPoolPalloc, we now call MemoryContextAlloc
and pfree instead of gp_malloc and gp_free, and we don’t do any memory
accounting.

So where does the performance improvement come from? Previously, we
would (essentially) pass in gp_malloc and gp_free to an underlying
allocation structure (which has been removed on the ORCA side). However,
we would add additional headers and overhead to maintain a list of all
of these allocations. When tearing down the memory pool, we would
iterate through the list of allocations and explicitly free each one. So
we would end up doing overhead on the ORCA side, AND the GPDB side.
However, the overhead on both sides was quite expensive!

If you want to compare against the previous implementation, see the
Allocate and Teardown functions in CMemoryPoolTracker.

With this PR, we improve optimization time by ~15% on average and up to
30-40% on some queries which are memory intensive.

This PR does remove memory accounting in ORCA. This was only enabled
when the optimizer_use_gpdb_allocators GUC was set. By setting
`optimizer_use_gpdb_allocators`, we still capture the memory used when
optimizing a query in ORCA, without the overhead of the memory
accounting framework.

Additionally, Add a top level ORCA context where new contexts are created

The OptimizerMemoryContext is initialized in InitPostgres(). For each
memory pool in ORCA, a new memory context is created in
OptimizerMemoryContext.
Co-authored-by: NShreedhar Hardikar <shardikar@pivotal.io>
Co-authored-by: NChris Hajas <chajas@pivotal.io>