Keepalive balancer module for nginx.

This module implements cache for backend connections.  As of now, it may
be used with memcached upstreams.  Support for fastcgi, http and https
requires expiremental patches for nginx.

Configuration directives:

    keepalive <num> [single]

        Scope: upstream

        Switches on keepalive module for the upstream in question.

        Parameters:

            - <num>
              Maximum number of connections to cache.  If there isn't enough
              room to cache new connections - last recently used connections
              will be kicked off the cache.

            - single
              Treat everything as single host.  With this flag connections
              to different backends are treated as equal.

Sample configuration:

    upstream memd {
        server 127.0.0.1:11211;
        server 10.0.0.2:11211;
        keepalive 10;
    }

This module was tested to work with standard round-robin balancing, but
it's believed to be compatible with more sophisticated balancers.  The only
requirement is to activate them *before* this module, e.g.:

    upstream memd {
        server 127.0.0.1:11211;
        server 10.0.0.2:11211;
        ip_hash;
        keepalive 10;
    }

To compile nginx with keepalive module, use "--add-module <path>" option to
nginx configure.