chg: dev: Use malloc_usable_size()/malloc_size() for memory accounting

Restore usage of malloc_usable_size()/malloc_size(), but this time only
for memory accounting and statistics purposes.  This should reduce the
memory footprint in case of compilation without jemalloc as we don't
have to keep track of the allocated memory size ourselves.

Merge branch 'ondrej/use-malloc_usable_size-when-available' into 'main'

See merge request isc-projects/bind9!11271