proposal: add external_acl flags: max_requests min_alive

I am using external_acl extensively in my production servers, and I
think it would be nice to add external_acl these flags:

o max_requests: after this number or requests, the helper will be
shutdown and replaced by a new one. This is to help leaky helpers.

o min_alive: to avoid short-lived helpers, they should stay alive at
least for this amount of seconds. This should help against
fork-query-kill per request behaviour if request rate is drastically
increased.

As a side note, only one helper would be replaced at a time, to avoid
replacing all of them at the same time.

defaults would be:
max_requests = no limit
min_alive = 1

Does this make sense?

Regards,

--
Gonzalo A. Arana