BUG/MINOR: pools: make sure to also destroy shared pools in pool_destroy_all()

BUG/MINOR: pools: make sure to also destroy shared pools in pool_destroy_all()

In issue #1677, Tim reported that we don't correctly free some shared
pools on exit. What happens in fact is that pool_destroy() is meant to
be called once per pool *pointer*, as it decrements the use count for
each pass and only releases the pool when it reaches zero. But since
pool_destroy_all() iterates over the pools list, it visits each pool
only once and will not eliminate some of them, which thus remain in the
list.

In an ideal case, the function should loop over all pools for as long
as the list is not empty, but that's pointless as we know we're exiting,
so let's just set the users count to 1 before the call so that
pool_destroy() knows it can delete and release the entry.

This could be backported to all versions (memory.c in 2.0 and older) but
it's not a real problem in practice.