filedescriptors, generalize that concept and use it for all cases
where we cannot accept and/or serve the connection:  (Lack of sessions,
lack of workerthreads).

This is controlled by three paramters:

Everytime we run into trouble, we increase the sleep-time by:
	acceptor_sleep_incr (0.001 s)

But we never let it get above
	acceptor_sleep_max  (0.050 s)

Once we manage to accept and schedule a connection, we multiply
the sleep-time by:
	acceptor_sleep_decay (0.9)

The default numbers are more or less picked out of thin air.

Two new stats counters help us keep track of this:
	accept_fail
		where accept(2) returns error.  This can be out of
		file-descriptors, but also clients which closed while
		they were stuck in the accept-queue.  Under normal
		operation, a minor trickle is probably to be expected.

	client_drop
		New connection dropped, because we could not get a
		session for it, or because the workerthreads were
		too busy.

	client_drop_late
		An previously served connection was dropped for the
		same reasons.

As always, feedback welcome.




git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4497 d4fa192b-c00b-0410-8231-f00ffab90ce4