OPTIM: pools: make pool_get_from_os() / pool_put_to_os() not update ->allocated

OPTIM: pools: make pool_get_from_os() / pool_put_to_os() not update ->allocated

The pool's allocation counter doesn't strictly require to be updated
from these functions, it may more efficiently be done in the caller
(even out of a loop for pool_flush() and pool_gc()), and doing so will
also help us spread the counters over an array later. The functions
were renamed _noinc and _nodec to make sure we catch any possible
user in an external patch. If needed, the original functions may easily
be reimplemented in an inline function.