Upstream: included backup peers into peer.tries.

Since peer.tries is never reset it can now be limited if required.
Upstream: limited next_upstream time and tries (ticket #544).

The new directives {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_tries
and {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_timeout limit
the number of upstreams tried and the maximum time spent for these tries
when searching for a valid upstream.