MdeModulePkg/DxeCore: use separate lock for pool allocations

MdeModulePkg/DxeCore: use separate lock for pool allocations

In preparation of adding memory permission attribute management to
the pool allocator, split off the locking of the pool metadata into
a separate lock. This is an improvement in itself, given that pool
allocations can only interfere with the page allocation bookkeeping
if pool pages are allocated or released. But it is also required to
ensure that the permission attribute management does not deadlock,
given that it may trigger page table splits leading to additional
page tables being allocated.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>