#4 'sticky' scheduler Mixin

[Anthony Baxter]

Implement the 'sticky' scheduler mixin. This should
cache the
last-used backend for a client address (for how long?)
and try
it first.

[Anthony Baxter]

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the client_addr is now passed to nextHost(). Next: implement
the cache, then the config directives, then make the manager
thread call the cache clear code.