MINOR: pools: prepare POOL_EXTRA to be split into multiple extra fields

MINOR: pools: prepare POOL_EXTRA to be split into multiple extra fields

Here the idea is to calculate the POOL_EXTRA size that is appended at
the end of a pool object based on the sum of enabled optional fields
so that we can more easily compute offsets and sizes depending on build
options.

For this, POOL_EXTRA is replaced with POOL_EXTRA_MARK which itself is
set either to sizeof(void*) or zero depending on whether we enable
marking the origin pool or not upon allocation.