MINOR: pools: report the amount used by thread caches in "show pools"

MINOR: pools: report the amount used by thread caches in "show pools"

The "show pools" command provides some "allocated" and "used" estimates
on the pools objects, but this applies to the shared pool and the "used"
includes what is currently assigned to thread-local caches. It's possible
to know how much each thread uses, so let's dump the total size allocated
by thread caches as an estimate. It's only done when pools are enabled,
which explains why the patch adds quite a lot of ifdefs.