Use erlang:demonitor(MRef, [flush])

Using the flush option removes a DOWN message matching the MRef from
the mailbox if present.
Make member start timeout configurable per pool

The time allowed for a member to start can be configured via
`{member_start_timeout, {1, min}}` in the per-pool config. The default
is one minute.
Do not schedule member culling if pool has fixed size

If init_count matches max_count, attempting to cull members will never
modify pool state. So avoid starting the timer and doing work for
nothing.
Enable stale member culling by default

New default behavior is to check for stale members once per minute
where max_age (stale member criteria) is 30 seconds.