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.
Access log: fixed the "if=" parameter with buffering (ticket #625).

It might not work if there were more than one "access_log" directives
pointed to the same file and duplicate buffer parameters.
SSL: session id context now includes certificate hash.

This prevents inappropriate session reuse in unrelated server{}
blocks, while preserving ability to restore sessions on other servers
when using TLS Session Tickets.

Additionally, session context is now set even if there is no session cache
configured.  This is needed as it's also used for TLS Session Tickets.

Thanks to Antoine Delignat-Lavaud and Piotr Sikora.
Updated OpenSSL used for win32 builds.