Keepalive balancer module for nginx.

This module implements cache for backend connections.  As of now, it may
be used with memcached upstreams.

Note: don't even try it with http backends.  It won't work.

Note: it's for nginx 0.7.* and up, won't work with nginx 0.6.*.

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.